Cathode follower without elevated heater voltage

[Dark Mavis]

I have an amp that's always had a problem. I noticed that it has a cathode follower stage, but the filaments are straight 6.3v directly out of the tranny. People talk about how cathode follower stages absolutely require elevated filament voltage, but they never say what the consequence of exceeding the heater-cathode voltage would be in terms of how the amp would sound/perform. Does anyone know? If it matches the way my amp behaves, then I think I'll have found the problem.

[nosaj]

I have an amp that's always had a problem. I noticed that it has a cathode follower stage, but the filaments are straight 6.3v directly out of the tranny. People talk about how cathode follower stages absolutely require elevated filament voltage, but they never say what the consequence of exceeding the heater-cathode voltage would be in terms of how the amp would sound/perform. Does anyone know? If it matches the way my amp behaves, then I think I'll have found the problem.

Please state your problem. You are asking us to pretty much guess. Here's some info on elevation. People here are like dogged reporters of old. They follow the facts, we follow the data you provide. We need problem, amp type and what your voltages for tubes are. Then you will most likely get better answers in a shorter time than if people have to ask for everything.

Welcome to the board and stay a while.
nosaj
from http://www.valvewizard.co.uk/heater.html
Heater Elevation
Elevation means referencing the heater supply to a DC voltage other than ground or zero volts. The heaters still operate at 6.3V or whatever, but this floats on top of the elevation voltage. Some valve stages such as cathode followers require the heater supply to be elevated to avoid exceeding the valve's Vhk(max) rating. But even when not explicitly needed, elevation can reduce hum in AC-heated circuits by reducing or saturating the leakage current between heater and cathode.*

The DC voltage is applied to a transformer centre tap, artificial centre tap, humdinger, or whatever reference connection the heater supply would normally have.

The elevation voltage can be taken from a potential divider across the HT (it doesn't matter where you position the divider), and an elevation voltage around 30 to 60V is typical. The divider should have a fairly high resistance so as not to waste current, although the lower arm (R2) should not be excessively large or Rhk(max) may be grossly exceeded, so it is advisable not to make it greater than 100k. The elevation voltage should be decoupled/smoothed with an arbitrarily large capacitor (C1), say 10uF or more.

Another convenient source of elevation voltage is the cathode of a cathode-biased power valve. No current flows ‘into’ the heater supply from here, so the power valve bias is not affected.

[Chuck H]

And to follow Jason's excellent briefing on the matter...

I'm not aware of any specific sonic consequences to filament/cathode over voltage conditions. Unless a failed tube and a busted amp is included So whatever problem you're having may be from a different cause (ergo Jason't request for more specifics on the actual problem).

Many classic amp designs exceed the heater to cathode maximum in one or more preamp stages. It was never a problem for over half a century. Problems with certain Rusky tubes have been noted over the years since. This problem is mostly worked out now and you don't hear about it much anymore with current production tubes. But no promises on that. The typical fix is to just use a Chinese (Shuguang) tube in the socket with the cathode follower.

[Dark Mavis]

Please state your problem. You are asking us to pretty much guess. Here's some info on elevation. People here are like dogged reporters of old. They follow the facts, we follow the data you provide. We need problem, amp type and what your voltages for tubes are. Then you will most likely get better answers in a shorter time than if people have to ask for everything.

Welcome to the board and stay a while.
nosaj
from http://www.valvewizard.co.uk/heater.html
Heater Elevation
Elevation means referencing the heater supply to a DC voltage other than ground or zero volts. The heaters still operate at 6.3V or whatever, but this floats on top of the elevation voltage. Some valve stages such as cathode followers require the heater supply to be elevated to avoid exceeding the valve's Vhk(max) rating. But even when not explicitly needed, elevation can reduce hum in AC-heated circuits by reducing or saturating the leakage current between heater and cathode.*

The DC voltage is applied to a transformer centre tap, artificial centre tap, humdinger, or whatever reference connection the heater supply would normally have.

The elevation voltage can be taken from a potential divider across the HT (it doesn't matter where you position the divider), and an elevation voltage around 30 to 60V is typical. The divider should have a fairly high resistance so as not to waste current, although the lower arm (R2) should not be excessively large or Rhk(max) may be grossly exceeded, so it is advisable not to make it greater than 100k. The elevation voltage should be decoupled/smoothed with an arbitrarily large capacitor (C1), say 10uF or more.

Another convenient source of elevation voltage is the cathode of a cathode-biased power valve. No current flows ‘into’ the heater supply from here, so the power valve bias is not affected.

I don't know what amp it is. It has no identifying marks or anything that suggests a manufacturer. The problem is that, beyond a certain (pretty low) volume, the amp sputters like a machine gun whenever notes are played. If I set the volume just on the threshold where things get weird, soft notes are fine, but hitting the strings a little harder, causes this crazy machine-gunning sound. The problem exists on both channels, and at around the same volume, regardless of whether the preamp is maxed and the master is low, or vice-versa. I tried playing in the dark, looking for signs of arcing in the tubes or on the sockets. I've tried swapping out the power tubes and preamp tubes. I've swapped out the output transformer. A guy who tried to fix it, said the signal is clean at the effects send, but I haven't confirmed this. The heaters are definitely not elevated. The preamp tubes have their own dedicated transformer, so I'll probably change that after the problem is sorted. Tubes are: 4XEL34s and 6 12AX7 (Actually, I think the driver tube was a 12AT7, but I haven't looked in this amp for a long time)
It's been suggested that it may be a lead dress problem. I don't have an oscilliscope, but I do have a multimeter. I did measure the plate voltages at one point, but I'd have to do it again, cus I don't remember. Like I said, it's been a while since I last had a crack at this amp. Last time I looked, I noticed a grid wire that never had the shielding grounded at either end, so I'll also need to sort that out to rule it out. I dunno if it would cause this problem though, but no harm in doing it.

[nosaj]

I don't know what amp it is. It has no identifying marks or anything that suggests a manufacturer. The problem is that, beyond a certain (pretty low) volume, the amp sputters like a machine gun whenever notes are played. If I set the volume just on the threshold where things get weird, soft notes are fine, but hitting the strings a little harder, causes this crazy machine-gunning sound. The problem exists on both channels, and at around the same volume, regardless of whether the preamp is maxed and the master is low, or vice-versa. I tried playing in the dark, looking for signs of arcing in the tubes or on the sockets. I've tried swapping out the power tubes and preamp tubes. I've swapped out the output transformer. A guy who tried to fix it, said the signal is clean at the effects send, but I haven't confirmed this. The heaters are definitely not elevated. The preamp tubes have their own dedicated transformer, so I guess the power trans can't provide enough current for all the tube's filaments (4XEL34s and 6 12AX7 (Actually, I think the driver tube was a 12AT7, but I haven't looked in this amp for a long time)
It's been suggested that it may be a lead dress problem. I don't have an oscilliscope, but I do have a multimeter. I did measure the plate voltages at one point, but I'd have to do it again, cus I don't remember. Like I said, it's been a while since I last had a crack at this amp. Last time I looked, I noticed a grid wire that never had the shielding grounded at either end, so I'll also need to sort that out to rule it out. I dunno if it would cause this problem though, but no harm in doing it.

Please post pics of the inside chassis and amp as a whole. Machine gun sounds could possibly be motorboating which would sugesst filter capacitors failing which makes sense based on your description as the caps cannot put out the required voltage when driven.

Pics will help for sure. If there is a dogbox will also need pics of caps in there.

Thanks,
nosaj

[Chuck H]

If the amp has six preamp tubes, a cathode follower and an effects loop it would be an ambitious build for diy. But with no manufacturer indicated this might be the case. So it might be an instability since diy builds of this order often suffer in areas that manufacturers have worked out having to do with lead dress and component layout. Since the amp has always had the problem I have to ask how it was acquired and if you know it's approximate age. That might help determine if the problem could be a failing filter capacitor.

Maybe you could rundown the control layout for us. We know there's a volume and a master volume. Is there an additional gain knob and the usual tone stack features? Multiple channels? Boost or bright switches, etc.

You should not start arbitrarily replacing things or even swapping out the OT, etc. In an amp like this it can only serve to complicate the issue. We see it all the time. Someone has no reason to replace or otherwise solder on some part of the amp and then things about the problem change and troubleshooting now involves two problems instead of one. Complicating things exponentially. Amps that suffer this kind of servicing commonly end up as dusty closet decor. Unless you know the preamp transformer is the cause of the problem you should not do anything to it. I will even add "please".

After you report on the control layout and/or provide some pics the next likely request will be to ask if anything other than low volume setting for the preamp or master affects the problem. That is, if you turn the treble up all the way, what happens? If you have the treble and the preamp volume up high? Does the amp make any noises at certain control settings with nothing plugged in? Do the tone controls affect the sound of the noise? That sort of thing. This will be an effort to isolate the problem and I hope you'll be gracious and play along.

[Dark Mavis]

[Chuck H]

Holy shit.

[Dark Mavis]

If the amp has six preamp tubes, a cathode follower and an effects loop it would be an ambitious build for diy. But with no manufacturer indicated this might be the case. So it might be an instability since diy builds of this order often suffer in areas that manufacturers have worked out having to do with lead dress and component layout. Since the amp has always had the problem I have to ask how it was acquired and if you know it's approximate age. That might help determine if the problem could be a failing filter capacitor.

Maybe you could rundown the control layout for us. We know there's a volume and a master volume. Is there an additional gain knob and the usual tone stack features? Multiple channels? Boost or bright switches, etc.

You should not start arbitrarily replacing things or even swapping out the OT, etc. In an amp like this it can only serve to complicate the issue. We see it all the time. Someone has no reason to replace or otherwise solder on some part of the amp and then things about the problem change and troubleshooting now involves two problems instead of one. Complicating things exponentially. Amps that suffer this kind of servicing commonly end up as dusty closet decor. Unless you know the preamp transformer is the cause of the problem you should not do anything to it. I will even add "please".

After you report on the control layout and/or provide some pics the next likely request will be to ask if anything other than low volume setting for the preamp or master affects the problem. That is, if you turn the treble up all the way, what happens? If you have the treble and the preamp volume up high? Does the amp make any noises at certain control settings with nothing plugged in? Do the tone controls affect the sound of the noise? That sort of thing. This will be an effort to isolate the problem and I hope you'll be gracious and play along.

So, apparently, it's built from a kit made by Torres Engineering. I have a schematic somewhere, but, if I remember, the schematic shows 2X 6L6 power tubes, rather than the 4XEL34 in this amp. The lead dress is horrendous. I'm considering pulling all the wiring out and re-doing it, just for the Hell of it. I bought this amp off the guy who put it together, and I can still get in touch with him if needs be. He demonstrated the problem to me before I even bought it. Yeah, it's a twin channel amp and has an effects loop and reverb. There seems to be a missing filter cap, but I vaguely remember needing it for something else. It's been a long time. I bought this amp almost 20 years ago. I even recorded an album with it and gigged with it for a few years. I just had to mic it up cus of the problem it has. It has a bunch of push-pull pots that seem to be bright switches and a gain boost? I don't know if they're part of the original design or not. I looked at the Torres web page, and this seems to be a Dual Marshall, and the description mentions various gimmicky switches.
IIRC the tone controls have no effect on this problem. Controls from left to right looking at the front of the amp, are: Channel 1 volume, treble, bass, mid. Channel 2 Gain, "Lead master", treble, bass, mid. Then there's reverb and then the master volume

[Dark Mavis]

Holy shit.

Yeah, every time I look at it, I remember the scene in lost boys where David offers Jason the noodles. I have room in the enclosure to actually extend the chassis by 2 inches backwards. I'm tempted to do that. Or maybe even fab a whole new chassis with more room, and mount the filter caps on the outside under a cover like on old Fenders to get more breathing space. I'm not sure I can tidy that wiring with so little space

[mozz]

" it would be an ambitious build"

I'll remember that description.

[Chuck H]

Yeah, every time I look at it, I remember the scene in lost boys where David offers Jason the noodles.

I think it was rice. And yeah. Believe it or not I was actually thinking "Torres mod" when I saw the pics.

Well the good news is that the amp has actually worked for the most part. I know I said that arbitrarily replacing parts was bad (Mkay), but after such a time and disuse it's probably a good idea to replace the filter capacitors (and put the missing cap back in). I know those IC caps get really grumpy with age and lack of charging in less than the age of this amp. So this would just be a matter of course to avoid complicating troubleshooting. That and some blowing out and cleaning, along with pot and contact cleaner would probably be the first order of business.

After that we could chase any instabilities. Dan Torres was typically very good about actually testing his circuits for the modular mod kits he sold. Curing instabilities in this case will likely involve adding HF bleeder circuit/s and hopefully that would be enough. Rewiring the whole thing may or may not actually improve circumstances. There's a lot of peripheral circuits to account for. Lot's of opportunity for error and in this case it may be a wish to get a more stable lead dress in the end. So if there's only a few snarky issues it might be easier to just address those if possible.

[Dark Mavis]

I think it was rice. And yeah. Believe it or not I was actually thinking "Torres mod" when I saw the pics.

Well the good news is that the amp has actually worked for the most part. I know I said that arbitrarily replacing parts was bad (Mkay), but after such a time and disuse it's probably a good idea to replace the filter capacitors (and put the missing cap back in). I know those IC caps get really grumpy with age and lack of charging in less than the age of this amp. So this would just be a matter of course to avoid complicating troubleshooting. That and some blowing out and cleaning, along with pot and contact cleaner would probably be the first order of business.

After that we could chase any instabilities. Dan Torres was typically very good about actually testing his circuits for the modular mod kits he sold. Curing instabilities in this case will likely involve adding HF bleeder circuit/s and hopefully that would be enough. Rewiring the whole thing may or may not actually improve circumstances. There's a lot of peripheral circuits to account for. Lot's of opportunity for error and in this case it may be a wish to get a more stable lead dress in the end. So if there's only a few snarky issues it might be easier to just address those if possible.

Rice first, then noodles. The rice looked like maggots and the noodles were worms. I'll start by replacing all the filter caps. The big grey ones were substitutions anyways, but I couldn't find enough lying around to sub them all. It's been at least 15 years since this thing was last plugged in, so I've forgotten a lot of stuff. I'll start by just replacing all the filter caps and reconnecting the output trans properly. I don't like seeing "electrical" tape. I found the missing filter cap, but I'll replace that, too. I see the resistor there is missing, too, so I gotta replace that as well

[Leo_Gnardo]

Holy shit.

Roger that ... looks like an explosion in the spaghetti factory!

[nosaj]

Roger that ... looks like an explosion in the spaghetti factory!

If by any chance please scan or attach a schematic. It will be very helpful.

nosaj