Cathode Resistor In Cathode Biased Amp Question

Tony-Cliffton

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1,251
Cathode resistor in 2x6v6 silvertone 1482. original is still in the amp and tests within spec, but voltage according to the schematic I have is supposed to be around 16v. We are getting 20. There is a good amount of crossover distortion at volume... heard as well as on the scope. Should this resistor be changed? I would like to hear less of the crossover distortion. I know it won't go away completely, but would like less. by change I mean change the value. Increase? Any advice would be great, thank you!
 

pdf64

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9,168
Could you provide audio clips / scope pic of the issue?
What are the mains, heater, and HT voltages, at idle and max clean(ish) output? Power tube idle dissipation?
I think the cathode resistor value would need reducing quite a bit to bring the voltage down that much; maybe the resistor is bad and failing under load?
 
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... voltage according to the schematic I have is supposed to be around 16v. We are getting 20. There is a good amount of crossover distortion at volume...

Probably one, or both, of the coupling caps into the 6V6s is leaking DC. That puts positive DC voltage on to the 6V6 grid and increases the bias current.
 
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Tony-Cliffton

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I'm no longer at my amp guys house, but I can tell you that the cathode bypass cap is new, as well as the cap can. We put one from CE manufacturing by Mallory in it. IIRC it a 450v 20/10/10. Is there a chance that can might be bad? Could we pull the tubes and set the meter for Vdc and check the grid pin to ground to see if there is leakage? Like I said the bypass cap is brand new as well. I will talk to him today. I know he was going to go through the preamp looking for bad couplers by pulling tubes (except rectifier) and check the grids for Vdc.
 

Tony-Cliffton

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Are you thinking the coupler from the PI to the output stage? I'm guessing so. He's going to check all the preamp couplers, I will report back. FWIW, the coupling caps are those green sangomo33 caps that are notorious for leakage.
On a side note, would you guys go with the Mallory 150m to replace the coupling caps? Or should I splurge on the sozo vintage? Or maybe the mojo polyester?
Sad to say, I'm into this amp for quite a bit of money, more than it's worth. I'm so deep now I have to finish it. I love the sound of it, so that's a bonus. I knew I shouldn't have bought a vintage amp:facepalm
 

Tony-Cliffton

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1,251
Yup, I'm using the schemo from Gary Mould. I know there are several schematics out there and it's confusing. Thank you for your help guys. I'm a novice at this stuff. I bias my own amps and know a fair amount, but don't solder well, otherwise I would be doing this myself. the first amp guy charged me $200 to do a three prong cord (which he didn't ground correctly) and change the filter cap can and bypass cap. I could hear things that weren't right like the crossover distortion and maybe blocking distortion, that's what led me to think coupling cap as well. so I brought it to this new guy who seems to know what he's doing and has a scope. We found and fixed the grounding, put shorting jacks in, found an osilation in the trem circuit. But what concerns me a bit is that I had to tell him how to check for a leaky cap? I don't think the first guy new either or he wouldn't have given it a "clean bill of health". Anyway, I'm rambling. thanks for your help guys.
 

UsableThought

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1,649
@Tony-Cliffton - one thing you might do to reduce the confusion: You've been given a very good suggestion to check wall voltages & the resulting internal voltages & also plate dissipation. When you get a chance, you can ask your amp guy if he's checked these & what they are, then report what he says here. That would help the two members here who trying to help you (not me). You might also check the wall voltage at your home to make sure it's reasonably close to that at your amp guy's location.
 
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Tony-Cliffton

Silver Supporting Member
Messages
1,251
@Tony-Cliffton - one thing you might do to reduce the confusion: You've been given a very good suggestion to check wall voltages & the resulting internal voltages & also plate dissipation. When you get a chance, you can ask your amp guy if he's checked these & what they are, then report what he says here. That would help the two members here who trying to help you (not me). You might also check the wall voltage at your home to make sure it's reasonably close to that at your amp guy's location.
Will do, thank you.
 
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826
... Could we pull the tubes and set the meter for Vdc and check the grid pin to ground to see if there is leakage? .... .

Yes, that's it. (Can do it with the tube in if you want.)

Are you thinking the coupler from the PI to the output stage? I'm guessing so. He's going to check all the preamp couplers, I will report back. FWIW, the coupling caps are those green sangomo33 caps that are notorious for leakage.
On a side note, would you guys go with the Mallory 150m to replace the coupling caps? Or should I splurge on the sozo vintage? Or maybe the mojo polyester?
Sad to say, I'm into this amp for quite a bit of money, more than it's worth. I'm so deep now I have to finish it. I love the sound of it, so that's a bonus. I knew I shouldn't have bought a vintage amp:facepalm

Yes. I'd go with the Mallory 150. A vintage amp will be great, once its sorted.
 

robrob

Member
Messages
408
Your particular power tube might be a hot runner too. Couple that with today's higher wall voltages and you can get to where you are. You might want to consider building (or having your amp guy build) a bucking transformer to drop down to vintage voltages.

https://robrobinette.com/5e3_Modifications.htm#Bucking_Transformer

Buckminister_Bucking_Transformer.png
 

rockometeramp

Member
Messages
432
Cathode resistor in 2x6v6 silvertone 1482. original is still in the amp and tests within spec, but voltage according to the schematic I have is supposed to be around 16v. We are getting 20. There is a good amount of crossover distortion at volume... heard as well as on the scope. Should this resistor be changed? I would like to hear less of the crossover distortion. I know it won't go away completely, but would like less. by change I mean change the value. Increase? Any advice would be great, thank you!


I can check on mine later, but if you're gonna change coupling caps, use the Orange Drop polyester

https://www.cedist.com/products/capacitors-orange-drop-600v-polyester

Silvertone used the old polyester Orange Drops when they weren't using the Sangamo or Mallory chocolate ones. They're priced right, and no snake oil. I have a 1474 that had the sangamos, almost every one was leaking DC.

Fwiw, I think the plate voltage on mine was around 345 and the voltage on the cathode was 19v(?) Ill check and post back later.
 

icr

Member
Messages
3,002
Cathode resistor in 2x6v6 silvertone 1482. original is still in the amp and tests within spec, but voltage according to the schematic I have is supposed to be around 16v. We are getting 20. There is a good amount of crossover distortion at volume... heard as well as on the scope. Should this resistor be changed? I would like to hear less of the crossover distortion. I know it won't go away completely, but would like less. by change I mean change the value. Increase? Any advice would be great, thank you!
Change it and see. The answer of anyone that does not have even a picture of the amp or a sound clip would be as reliable as this:
Possibilities-Tarot-Card-Spread.png
 

Tony-Cliffton

Silver Supporting Member
Messages
1,251
I can check on mine later, but if you're gonna change coupling caps, use the Orange Drop polyester

https://www.cedist.com/products/capacitors-orange-drop-600v-polyester

Silvertone used the old polyester Orange Drops when they weren't using the Sangamo or Mallory chocolate ones. They're priced right, and no snake oil. I have a 1474 that had the sangamos, almost every one was leaking DC.

Fwiw, I think the plate voltage on mine was around 345 and the voltage on the cathode was 19v(?) Ill check and post back later.
Thanks for your input Rocco! I will go with the OD polyester. Haven't heard from him yet. We had a pretty good snow last night/this morning so he is probably shoveling or plowing. Just got done myself. Ugh, winter.
 

rockometeramp

Member
Messages
432
Plates on mine with 120v from the wall were 361vdc, cathode was 19.6 or so. I got almost identical readings on a friends 1482 that just happened to be in my basement.

If you look on older Fender schematics and layouts, they show a + or - 20v variance with 117v at the wall. So, 16v +20% is 19.2v. Add higher wall voltages and you get your 20v on the cathode. If you have a variac, check it at 117v and you'll probably be closer to 16v on the cathode.

One thing you want to make sure of is using the right secondary tap on the OT. The original speaker was a 4 ohm, and the OT has 4&8 ohm taps, the 8 ohm tap is connected to the terminal strip already.I can't say I know what crossover distortion sounds like, but they don't sound good using the wrong tap.
 
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826
The cathode voltage on a cathode-biased amp is self-regulating to some extent, so I don't think it should vary as much with wall voltage as (for example) the plate voltage does.
 

Tony-Cliffton

Silver Supporting Member
Messages
1,251
Plates on mine with 120v from the wall were 361vdc, cathode was 19.6 or so. I got almost identical readings on a friends 1482 that just happened to be in my basement.

If you look on older Fender schematics and layouts, they show a + or - 20v variance with 117v at the wall. So, 16v +20% is 19.2v. Add higher wall voltages and you get your 20v on the cathode. If you have a variac, check it at 117v and you'll probably be closer to 16v on the cathode.

One thing you want to make sure of is using the right secondary tap on the OT. The original speaker was a 4 ohm, and the OT has 4&8 ohm taps, the 8 ohm tap is connected to the terminal strip already.I can't say I know what crossover distortion sounds like, but they don't sound good using the wrong tap.
Yeah, hes changing the tap over to the 8ohm. Im sure your're right about the voltage. Have to see if there is any bad couplers found. That could really change things. Im sure there are caps leaking. The track record on those sangamos isn't very promising. I will have to report back tomorrow, as i think he may be a bit busy today. I told him i really need the amp by mid of next week. I really apreciate your help/guidance guys. Thanks for the voltages rocco, i will give them to him when we speak.
 

Tony-Cliffton

Silver Supporting Member
Messages
1,251
Plates on mine with 120v from the wall were 361vdc, cathode was 19.6 or so. I got almost identical readings on a friends 1482 that just happened to be in my basement.

If you look on older Fender schematics and layouts, they show a + or - 20v variance with 117v at the wall. So, 16v +20% is 19.2v. Add higher wall voltages and you get your 20v on the cathode. If you have a variac, check it at 117v and you'll probably be closer to 16v on the cathode.

One thing you want to make sure of is using the right secondary tap on the OT. The original speaker was a 4 ohm, and the OT has 4&8 ohm taps, the 8 ohm tap is connected to the terminal strip already.I can't say I know what crossover distortion sounds like, but they don't sound good using the wrong tap.
Rocco, if you dont know what crossover distortion sounds like, here is an example. At the 40 sec to like 1min mark in the video best represents the sound. Mine isnt quite that bad but you can really hear it on sustained notes with the amp running at a good clip. Having the speaker on the 4ohm tap wouldn't cause this. Its pretty much a bias thing. Could be pre amp or power. Anyway, now you know what crossover distortion sounds like. All "tweed" style amps have it to som extent, and some can actually sound ok. When it gets out of hand, well, you heard the video. On a scope it will look like a "notch" or a bump or kink right when the wave crosses the base line. Hence, crossover in the name.
 

rockometeramp

Member
Messages
432
Sounded fine til the pedal came on! I have to say I still don't know crossover distortion when I hear it, and I probably have, I just dont know it.
 

Tony-Cliffton

Silver Supporting Member
Messages
1,251
Plates on mine with 120v from the wall were 361vdc, cathode was 19.6 or so. I got almost identical readings on a friends 1482 that just happened to be in my basement.

If you look on older Fender schematics and layouts, they show a + or - 20v variance with 117v at the wall. So, 16v +20% is 19.2v. Add higher wall voltages and you get your 20v on the cathode. If you have a variac, check it at 117v and you'll probably be closer to 16v on the cathode.

One thing you want to make sure of is using the right secondary tap on the OT. The original speaker was a 4 ohm, and the OT has 4&8 ohm taps, the 8 ohm tap is connected to the terminal strip already.I can't say I know what crossover distortion sounds like, but they don't sound good using the wrong tap.
Ok heard back from amp guy. Rocco, gave him the voltages and hes going to test when hes done, at the wall, as well on a variac. We are pretty confident it will be fine. He said he would test a couple different values for the cathode resistor just to see if he can optimize the bias. Right now at hand was the leaky couplers. All of them were leaking, some more than others. So, we are changing them, thats where i think my crossover distortion/blocking distortion was coming from, the bias of the preamp tubes. He had the mallory 150m's in stock so we are using those. he's using the outer foil to plate method. While he is doing that , he installed polystyrene .003 caps for the caps to the tone controls. Kind of an experiment to see how they work here. This amp will be used for some recording at some point so we changed the input wire off the jacks to shielded wire. Installed shorting jacks on all inputs including trem. Didnt change any resistors, anywhere. Also did a star type ground, it lowered the noise floor quite a bit, so much that he doesn't feel we need to do an artificial center tap on the 6.3v. Maybe later, have to play it at my house.
Other things i did to the amp are: (in order to the best of my memory)
Electrolytic cap change
Three prong cord with upgraded grounding
Brand new hand crafted, bigger pine cab made by Rocco at Rockometer.com who did a phenominal job with the cab. He also has the cabs in stock, and uncovered ready to go. Less than a week tunaround, super easy, no pay pal. Takes credit/debit over the phone (yes, you get to talk to him!) as you can see, he's been helpful here! Thanks again, Rocco.
The cab made a huge difference. Probaly the most drastic change out of anything i've done (between the filter cap change and whats getting done now) so far. Much bigger, clearer sound.
Speaker change: on this one, the old speaker worked, but there was noise on low notes when pushed, even after the filer cap change. Thought it was mechanical. Took speaker out, cone had a small tare. Enter new weber sig12a...didnt fix the noise, either did the cab. Changing to 8ohm tap with new speaker. I still have the original speaker as well as the original cabinet, and will save all the old parts. Not sure who would want them but, I got em.
Tubes: had good old rca grey plate 6v6gt, long plate [] getter grey plate rca 12ax7 preamp, and a what looks like either a ge or cbs short grey plate 12ax7. Anyway, took the originals out and got a nos 50's RCA long black plate 7025, an now bugleboy 6au6, a mullard 6x4 (i think its a mullard) and new production GT6v6gt's (probably put the old ones back in, got these for troubleshooting, but they sound pretty good!) Anyway, all new tubes but still that nasty noise on the low notes, like blocking distortion, as well as a bunch of crossover distortion. So now im thinking bias, which led me to bring it to this new guy, who i really, really like so far. Great communication and puts up with my obsessiveness.
So he is now doing what i mentioned above. Beyond that i dont have anything else.
I know changing all the caps might not make it "vintage" or "original" anymore, but i want an amp i can play and record, and have fun with.... A 1959 bassmen, its not. The circuit is exactly the same. Compared to what some people do as far as mods to these amps, when they are done its not even the same amp. I'm not trying to make this amp into something it's not, I love the sound of them. All i have done is tried to eliminate noise and wasted power, which will make it run more efficiently, thus more tone, a little head room, and with any luck a little sparkle, and more dynamics. I cant wait to get it back! Got a nice treble booster waiting to hammer it with. :D
 
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pdf64

Member
Messages
9,168
Rocco, if you dont know what crossover distortion sounds like, here is an example. At the 40 sec to like 1min mark in the video best represents the sound. Mine isnt quite that bad but you can really hear it on sustained notes with the amp running at a good clip
Crossover distortion doesn't tend to sound like that clip.
My experience is that the clip better represents blocking distortion.
 




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