Feel free to ask Myles

[Professor Monkey]

Would you suggest buying a strap lock?

 

If so, which brand?

[myles_rose]

Originally posted by SlvrDragon50:

Would you suggest buying a strap lock?

 

If so, which brand?

That is personal preference and there are many good ones on the market.

[WaterMan]

Myles,

 

You've indiated in another post that one of your favorite setups is a 12ay7 in V1 with a 6V6 as output tube. My question is this: doesn't a low output preamp tube like a 12ay7 result in less power tube overdrive and more preamp distortion, since a lower power preamp tube like the 12ay7 will break up earlier and send less output to the power tubes? If this is correct (which it certainly may not be), how does this correlate to most blues players prefering more of the warmth associated with pushing the power tubes and less preamp distortion?

 

Thanks for helping me sort all this out.

[Junior 1]

Myles,

I bought a not working Epiphone Valve Special 5 watt amp today at the local GC. The 10 inch speaker works fine with my VJ head, but when I power up the Special, the tubes seem to light, but I get a low hum, some crakle, and some screeching! Nothing from the guitar. What do you think? I'm not going to be heart broken if it's DOA, I wanted the cab and speaker for my VJ head.

Thanks

[myles_rose]

Originally posted by WaterMan:

Myles,

 

You've indiated in another post that one of your favorite setups is a 12ay7 in V1 with a 6V6 as output tube. My question is this: doesn't a low output preamp tube like a 12ay7 result in less power tube overdrive and more preamp distortion, since a lower power preamp tube like the 12ay7 will break up earlier and send less output to the power tubes? If this is correct (which it certainly may not be), how does this correlate to most blues players prefering more of the warmth associated with pushing the power tubes and less preamp distortion?

 

Thanks for helping me sort all this out.

There are a number of factors in preamp tubes that come into play such as gain and current which are two different things.

 

A 12AT7 as an example has a current output at driven rating of 10mA, almost ten times more than a 12AX7. A 12AY7 has a gain of 40 vs 100 with a current output almost 3x of a 12AX7 as another example.

 

A preamp tube with lower gain will drive an output section to it's design level at a different ratio than the higher gain tube.

[myles_rose]

Originally posted by Junior 1:

Myles,

I bought a not working Epiphone Valve Special 5 watt amp today at the local GC. The 10 inch speaker works fine with my VJ head, but when I power up the Special, the tubes seem to light, but I get a low hum, some crakle, and some screeching! Nothing from the guitar. What do you think? I'm not going to be heart broken if it's DOA, I wanted the cab and speaker for my VJ head.

Thanks

GC should fix it unless it was sold "as is". Contact the store manager, not the salesman.

 

It may just be the 12AX7 or single EL84 so it may be easier to change those out first. If one of those are the issue then GC should replace the offending tube with a new one.

[WaterMan]

Originally posted by myles111:

Originally posted by WaterMan:

Myles,

 

You've indiated in another post that one of your favorite setups is a 12ay7 in V1 with a 6V6 as output tube. My question is this: doesn't a low output preamp tube like a 12ay7 result in less power tube overdrive and more preamp distortion, since a lower power preamp tube like the 12ay7 will break up earlier and send less output to the power tubes? If this is correct (which it certainly may not be), how does this correlate to most blues players prefering more of the warmth associated with pushing the power tubes and less preamp distortion?

 

Thanks for helping me sort all this out.

There are a number of factors in preamp tubes that come into play such as gain and current which are two different things.

 

A 12AT7 as an example has a current output at driven rating of 10mA, almost ten times more than a 12AX7. A 12AY7 has a gain of 40 vs 100 with a current output almost 3x of a 12AX7 as another example.

 

A preamp tube with lower gain will drive an output section to it's design level at a different ratio than the higher gain tube.

Thanks Myles. Just so I fully understand (simplistically speaking), in the case of the 12AY7, lower gain means less pre-amp distortion than a 12AX7, and the higher current output means a stronger signal (more "drive") to the power tubes?

[myles_rose]

Originally posted by WaterMan:

Originally posted by myles111:

Originally posted by WaterMan:

Myles,

 

You've indiated in another post that one of your favorite setups is a 12ay7 in V1 with a 6V6 as output tube. My question is this: doesn't a low output preamp tube like a 12ay7 result in less power tube overdrive and more preamp distortion, since a lower power preamp tube like the 12ay7 will break up earlier and send less output to the power tubes? If this is correct (which it certainly may not be), how does this correlate to most blues players prefering more of the warmth associated with pushing the power tubes and less preamp distortion?

 

Thanks for helping me sort all this out.

There are a number of factors in preamp tubes that come into play such as gain and current which are two different things.

 

A 12AT7 as an example has a current output at driven rating of 10mA, almost ten times more than a 12AX7. A 12AY7 has a gain of 40 vs 100 with a current output almost 3x of a 12AX7 as another example.

 

A preamp tube with lower gain will drive an output section to it's design level at a different ratio than the higher gain tube.

Thanks Myles. Just so I fully understand (simplistically speaking), in the case of the 12AY7, lower gain means less pre-amp distortion than a 12AX7, and the higher current output means a stronger signal (more "drive") to the power tubes?

It is more of a matter of which side of the amp ... preamp or poweramp, is driven into distortion earlier.

[jmp1]

Apologies for the length of this post - I hope you can bear with me .....

 

I am planning to revive a Marshall JTM45 MkII and would very grateful for any information, advice or comments.

 

I believe that the amp in question is a '60s JTM45 MK II head - based on the following ......

 

There is a silver "coffin style" Marshall name plate with scroll edges and enamelled wine red block lettering. It has a leather carry handle on the top.

 

The front control panel is self-coloured thin ally sheet. "JTM45" appears at the extreme right hand edge and "MK II" at the left hand edge. There are two toggle switches - one marked "OFF ON", the other "STANDBY ON" - there is no polarity switch. Potentiometer controls have round dark brown bakelite knobs with milled edges.

 

The rear panel is a white laminated "melamine" hard plastic sheet with engraved labels. It has "BASS SER2300" engraved, and, dead centre, there is a single jack socket labelled "SPEAKER".

 

The chassis is a folded aluminium "tray" which is smaller than the width of the cabinet (looking from the front).

 

There are six valves - 3 x ECC83 (1 Mazda, 2 anonymous), 1 x GZ34 (Mazda) and 2 x EL34 (Mullard) (more on this later).

 

The choke is an RS item labelled "HYGRADE L.F. CHOKE 20 HENRYS, 70 mAs. 690 O" (where O is the omega ohms symbol).

 

The output transformer is an RS item labelled "DE LUXE OUTPUT TRANSFORMER" - it does not have an impedance selector.

 

I believe the power transformer is an RS item - the only obvious marking is a small dab of white paint on the wrapping.

 

The circuit board is a reddish-brown perforated "mica" breadboard. The circuit uses solder turrets and point-to-point wiring.

 

There is no bias current trimmer/ preset potentiometer.

 

There is no "500mA slow blow fuse", as is shown on some schematics.

 

Comparing the circuit with a number of schematics, I reckon that the amp would probably originally have been fitted with KT-66s rather than EL34s.

 

Going on physical appearance of components, solder joints, etc. and comparison with schematics and photos, the rest of the circuitry looks original to me except for one item. Within the bias power supply circuit, the resistor connecting the cathodes of the two 8uF electrolytics is 320k (colour bands orange-red-yellow and confirmed with meter measurement) rather than the 15k which appears in various schematics and photos I have seen.

 

Assuming that this 320k is not original, could it be a modification in order to bias the EL34s? I would be very interested learn whether this is a surprise or a known "tweak". (I did wonder whether someone had read the colour bands in the wrong direction and believed they were using a 42k resistor.)

 

Speaker-wise, the amp is currently mated to an 8 ohm cabinet.

 

However, my current conclusion is that the amp was designed to drive a 16 ohm cabinet using KT-66s. But I have also read that the nominal output impedance should be halved for EL34s compared with KT-66s, so that, if correct, 8 ohm would in fact be appropriate for the EL34s.

 

I have had the amp working briefly, but there seems to be quite a lot of hum. It also, subjectively, seems to get pretty hot.

 

Ideally, I would like initially to return to something like the original (electronic) spec, prior to any future adventures such as different tubes or tweaking to a "lead" rather than bass configuration. Cosmetically, whilst the amp looks well travelled, this is part of its character and I don't have any plans to attempt a "showroom" face-lift.

 

In terms of work to embark on initially, I am considering the following:

 

- Replace all electroytics (some are showing signs of distortion or leakage)

 

- Replace wobbly toggle switches

 

- Replace mains cable

 

- EITHER Replace EL34s with KT-66 and get a 16 ohm speaker cab,

 

- OR, If staying with EL34, replace the existing 180k resistor on the cathode side of the bias supply diode with a 220k item, in line with schematics for later EL34 equipped Marshall models. Stick with the 8 ohm cab,

 

- Replace the strange 320k resistor in the bias circuit with 15k, in line with various photos and schematics.

 

- Add bias adjustment by replacing the existing 56k resistor in the bias circuit with a 25k 0.5W cermet trimmer and a 47K resistor in series.

 

- Add 1 or 10 ohm "bias measurement" resistors between pin 8 and ground of the power valve sockets (how large a value could in fact be used before it would significantly affect the operation of the amp?)

 

- Check grounding

 

- Adjust the bias current appropriate to the chosen tubes (any suggestions on suitable values for EL34 and KT66?)

 

- Obtain some form of power attenuator/ soak, to avoid having to glue all the ornaments to the mantelpiece

 

- Compare voltages against expected values, based on circuit schematics

 

- Turn on, warm up, plug in and get on down!

 

 

Anyway, once again apologies for the length of this post - any advice or comment will be gratefully received.

[myles_rose]

Originally posted by jmp1:

Apologies for the length of this post - I hope you can bear with me .....

 

I am planning to revive a Marshall JTM45 MkII and would very grateful for any information, advice or comments.

 

I believe that the amp in question is a '60s JTM45 MK II head - based on the following ......

 

There is a silver "coffin style" Marshall name plate with scroll edges and enamelled wine red block lettering. It has a leather carry handle on the top.

 

The front control panel is self-coloured thin ally sheet. "JTM45" appears at the extreme right hand edge and "MK II" at the left hand edge. There are two toggle switches - one marked "OFF ON", the other "STANDBY ON" - there is no polarity switch. Potentiometer controls have round dark brown bakelite knobs with milled edges.

 

The rear panel is a white laminated "melamine" hard plastic sheet with engraved labels. It has "BASS SER2300" engraved, and, dead centre, there is a single jack socket labelled "SPEAKER".

 

The chassis is a folded aluminium "tray" which is smaller than the width of the cabinet (looking from the front).

 

There are six valves - 3 x ECC83 (1 Mazda, 2 anonymous), 1 x GZ34 (Mazda) and 2 x EL34 (Mullard) (more on this later).

 

The choke is an RS item labelled "HYGRADE L.F. CHOKE 20 HENRYS, 70 mAs. 690 O" (where O is the omega ohms symbol).

 

The output transformer is an RS item labelled "DE LUXE OUTPUT TRANSFORMER" - it does not have an impedance selector.

 

I believe the power transformer is an RS item - the only obvious marking is a small dab of white paint on the wrapping.

 

The circuit board is a reddish-brown perforated "mica" breadboard. The circuit uses solder turrets and point-to-point wiring.

 

There is no bias current trimmer/ preset potentiometer.

 

There is no "500mA slow blow fuse", as is shown on some schematics.

 

Comparing the circuit with a number of schematics, I reckon that the amp would probably originally have been fitted with KT-66s rather than EL34s.

 

Going on physical appearance of components, solder joints, etc. and comparison with schematics and photos, the rest of the circuitry looks original to me except for one item. Within the bias power supply circuit, the resistor connecting the cathodes of the two 8uF electrolytics is 320k (colour bands orange-red-yellow and confirmed with meter measurement) rather than the 15k which appears in various schematics and photos I have seen.

 

Assuming that this 320k is not original, could it be a modification in order to bias the EL34s? I would be very interested learn whether this is a surprise or a known "tweak". (I did wonder whether someone had read the colour bands in the wrong direction and believed they were using a 42k resistor.)

 

Speaker-wise, the amp is currently mated to an 8 ohm cabinet.

 

However, my current conclusion is that the amp was designed to drive a 16 ohm cabinet using KT-66s. But I have also read that the nominal output impedance should be halved for EL34s compared with KT-66s, so that, if correct, 8 ohm would in fact be appropriate for the EL34s.

 

I have had the amp working briefly, but there seems to be quite a lot of hum. It also, subjectively, seems to get pretty hot.

 

Ideally, I would like initially to return to something like the original (electronic) spec, prior to any future adventures such as different tubes or tweaking to a "lead" rather than bass configuration. Cosmetically, whilst the amp looks well travelled, this is part of its character and I don't have any plans to attempt a "showroom" face-lift.

 

In terms of work to embark on initially, I am considering the following:

 

- Replace all electroytics (some are showing signs of distortion or leakage)

 

- Replace wobbly toggle switches

 

- Replace mains cable

 

- EITHER Replace EL34s with KT-66 and get a 16 ohm speaker cab,

 

- OR, If staying with EL34, replace the existing 180k resistor on the cathode side of the bias supply diode with a 220k item, in line with schematics for later EL34 equipped Marshall models. Stick with the 8 ohm cab,

 

- Replace the strange 320k resistor in the bias circuit with 15k, in line with various photos and schematics.

 

- Add bias adjustment by replacing the existing 56k resistor in the bias circuit with a 25k 0.5W cermet trimmer and a 47K resistor in series.

 

- Add 1 or 10 ohm "bias measurement" resistors between pin 8 and ground of the power valve sockets (how large a value could in fact be used before it would significantly affect the operation of the amp?)

 

- Check grounding

 

- Adjust the bias current appropriate to the chosen tubes (any suggestions on suitable values for EL34 and KT66?)

 

- Obtain some form of power attenuator/ soak, to avoid having to glue all the ornaments to the mantelpiece

 

- Compare voltages against expected values, based on circuit schematics

 

- Turn on, warm up, plug in and get on down!

 

 

Anyway, once again apologies for the length of this post - any advice or comment will be gratefully received.

I am sorry but due to the length of this ....

 

what is the question?

 

Have you looked on my GAB website as there is a lot of JTM-45 info and links to prints and specs.

[Paul24]

Hi, It was my intention to take the lineout on my Rivera Pubster 25 into my Yamaha AG16 Hi Z input hopefully recording what was going through the amp but these ideas were somewhat smashed when I can only get about 2/3rds of the signal needed for my Yamaha .If I try pushing the input volume I get hiss (not good for recording !)This is a bit of a blow as i sold my Fender deville 4x10 for the Rivera for recording.....

Any help/ideas

Thanks

Paul

[Paul24]

Hi, It was my intention to take the lineout on my Rivera Pubster 25 into my Yamaha AG16 Hi Z input hopefully recording what was going through the amp but these ideas were somewhat smashed when I can only get about 2/3rds of the signal needed for my Yamaha .If I try pushing the input volume I get hiss (not good for recording !)This is a bit of a blow as i sold my Fender deville 4x10 for the Rivera for recording.....

Any help/ideas

Thanks

Paul

[Paul24]

Hi, It was my intention to take the lineout on my Rivera Pubster 25 into my Yamaha AG16 Hi Z input hopefully recording what was going through the amp but these ideas were somewhat smashed when I can only get about 2/3rds of the signal needed for my Yamaha .If I try pushing the input volume I get hiss (not good for recording !)This is a bit of a blow as i sold my Fender deville 4x10 for the Rivera for recording.....

Any help/ideas

Thanks

Paul

[myles_rose]

Originally posted by Paul24:

Hi, It was my intention to take the lineout on my Rivera Pubster 25 into my Yamaha AG16 Hi Z input hopefully recording what was going through the amp but these ideas were somewhat smashed when I can only get about 2/3rds of the signal needed for my Yamaha .If I try pushing the input volume I get hiss (not good for recording !)This is a bit of a blow as i sold my Fender deville 4x10 for the Rivera for recording.....

Any help/ideas

Thanks

Paul

Normally the Rivera line out is plenty hot enough to drive most anything. I am not up on your Yamaha unit's specs or requirements but if there is a line/instrument level switch or settings to edit within the Yamaha unit that is what I would try first.

[Paul24]

Thank for this Myles,

 

The Rivera output shows 560 ohms ..

 

The Yamaha Hi-Z input (from spec) states Input impedance 500 k ohms.

 

The other channels show Input impedance 3k ohms..

 

Sorry but this doesn't mean much to me.

 

Thanks

 

Paul

[myles_rose]

Originally posted by Paul24:

Thank for this Myles,

 

The Rivera output shows 560 ohms ..

 

The Yamaha Hi-Z input (from spec) states Input impedance 500 k ohms.

 

The other channels show Input impedance 3k ohms..

 

Sorry but this doesn't mean much to me.

 

Thanks

 

Paul

As I said earlier, this should work fine and dandy, there may be some level setting parameters in the Yamaha unit that need to be set.

[Junior 1]

Hi Myles,

I have a chance to pick up a Squier Bronco short scale bass. These are student basses with one strat pickup. Can I use it with my Valve Junior Combo?

[myles_rose]

Originally posted by Junior 1:

Hi Myles,

I have a chance to pick up a Squier Bronco short scale bass. These are student basses with one strat pickup. Can I use it with my Valve Junior Combo?

I would not do this as the speaker is not designed for those frequencies.

[dr_jeffers]

Hi Myles,

 

I bought a Rivera Quiana Studio about a week back. Lovely amp. It has a modern/vintage rocker switch that allows the 55W rating to be roughly halved (my understanding is limited, but presumably by switching to just one 6L6 tube?). The manual states that operation is switched from pentode (modern) to triode (vintage). I have noticed that doing this causes a significant hum when the standby switch is flipped (ie. no hum when in standby mode). The level of this hum is unaffected by switching channels or any of the master or preamp volume controls. Sounds like a mains hum to me. Any ideas what might be causing this, and how it might be done away with- its annoying, as the low-volume advantage of the triode mode is counter-acted by the loud hum, which mars low volume playing. Oddly, I don't remember it doing this when i extensively demo'd it at the dealers- i'm sure i would have noticed! I'm wondering if its something to do with my home circuits? Incidently, i am in the UK, and therefore have UK (250V) version of the amp. Based on my (quite possibly flawed!) understanding that only one tube is driving in vintage mode, i tried swapping round the two 6L6s to see if that made any difference- none. I also made sure there was nothing else operating from the mains (e.g effects transformer) to interfere.

 

There is also a much quieter (live-able-with) mains hum when operating in modern mode (in either standby or 'go' mode). Not a problem, but is this normal? I was under the impression that high-end Riveras were pretty whisper-like.

 

Any ideas about how to remove the hum would be appreciated! Its a long trip back to the dealer! And many thanks in advance for the benefit of your expertise.

[myles_rose]

Originally posted by dr_jeffers:

Hi Myles,

 

I bought a Rivera Quiana Studio about a week back. Lovely amp. It has a modern/vintage rocker switch that allows the 55W rating to be roughly halved (my understanding is limited, but presumably by switching to just one 6L6 tube?). The manual states that operation is switched from pentode (modern) to triode (vintage). I have noticed that doing this causes a significant hum when the standby switch is flipped (ie. no hum when in standby mode). The level of this hum is unaffected by switching channels or any of the master or preamp volume controls. Sounds like a mains hum to me. Any ideas what might be causing this, and how it might be done away with- its annoying, as the low-volume advantage of the triode mode is counter-acted by the loud hum, which mars low volume playing. Oddly, I don't remember it doing this when i extensively demo'd it at the dealers- i'm sure i would have noticed! I'm wondering if its something to do with my home circuits? Incidently, i am in the UK, and therefore have UK (250V) version of the amp. Based on my (quite possibly flawed!) understanding that only one tube is driving in vintage mode, i tried swapping round the two 6L6s to see if that made any difference- none. I also made sure there was nothing else operating from the mains (e.g effects transformer) to interfere.

 

There is also a much quieter (live-able-with) mains hum when operating in modern mode (in either standby or 'go' mode). Not a problem, but is this normal? I was under the impression that high-end Riveras were pretty whisper-like.

 

Any ideas about how to remove the hum would be appreciated! Its a long trip back to the dealer! And many thanks in advance for the benefit of your expertise.

The pentode/triode does not cut out one tube, it changes the operation of the tube from using five internal elements of the tube to three.

 

I would verify that the bias is properly set and that the output tubes are matched. It is not uncommon for output tubes to go out of match from shipping or handling damage. Mismatched output tubes will make an amp hum and they could be fine in one mode and not in other, hence your problem.

[volusia1]

Hi, I just completed a new project called "Never Know" over the weekend. It is a guitar driven instumental. I also managed to finally get a myspace page going. It's taking me forever to figure out my recorder now I have to figure out myspace too. LOL I would appreciate any constructive criticism or pointers. This is still a new experiance for me as I have only been playing for 2 years.

Thanks,

 

http://www.myspace.com/brianlucasblues

[myles_rose]

Originally posted by volusia1:

Hi, I just completed a new project called "Never Know" over the weekend. It is a guitar driven instumental. I also managed to finally get a myspace page going. It's taking me forever to figure out my recorder now I have to figure out myspace too. LOL I would appreciate any constructive criticism or pointers. This is still a new experiance for me as I have only been playing for 2 years.

Thanks,

 

http://www.myspace.com/brianlucasblues

There are other places to post this but if you want me to review something send it as an attached file to my GAB email address

[Junior 1]

Hi Myles,

I'm a little delayed, but back on 1/11, I asked if I could use my Valve Junior with a short scale bass. You answered my question, thank you. Could I install a bass speaker in that amp and then use it?

[myles_rose]

Originally posted by Junior 1:

Hi Myles,

I'm a little delayed, but back on 1/11, I asked if I could use my Valve Junior with a short scale bass. You answered my question, thank you. Could I install a bass speaker in that amp and then use it?

You could do that but why have two amps in one .... get a 1x12 cab or 1x15 cab and put in a bass speaker and then just plug that cab into the amp instead of the internal speaker if the amp is a combo and not the head.

[Junior 1]

Thanks, Myles. I think I'll do just that! Any excuse for GAS.

[Big Red 67]

Hey Myles! Are the Bogen CT-35's tube amp PA's?

[myles_rose]

Originally posted by Big Red 67:

Hey Myles! Are the Bogen CT-35's tube amp PA's?

I do not remember but most of the Bogen PAs were tube units.

[jross68052]

Myles,

 

Quick question. I ususally run my Prosonic amp with the Class A rectifier setting. My understanding is that in Class A it creates 30 Watts. This gives me a good usable amount of power for club gigs. My question is:

 

Do I need a balanced phase inverter tube for this mode of operation?

 

Thanks,

 

Joe

[myles_rose]

Originally posted by jross68052:

Myles,

 

Quick question. I ususally run my Prosonic amp with the Class A rectifier setting. My understanding is that in Class A it creates 30 Watts. This gives me a good usable amount of power for club gigs. My question is:

 

Do I need a balanced phase inverter tube for this mode of operation?

 

Thanks,

 

Joe

You amp is push-pull regardless of cathode bias or grid bias .... each half of the phase inverter pushes one output tube. I use a MPI in any amp of this type no matter the setting of the amp.

[Big Red 67]

Originally posted by myles111:

Originally posted by Big Red 67:

Hey Myles! Are the Bogen CT-35's tube amp PA's?

I do not remember but most of the Bogen PAs were tube units.

Cool Thanks.