Peavey Classic 30 Rebuild Cont.

[Opry Audio]

Hey all,

I have a previous thread about this amp but wanted to touch base about it on another one since it’s in a different stage of functioning. The amp overall sounds good but I’m working through some noise and hum issues and hoping to get some help here. Thank you to everyone who’s answered my questions so far. Here’s the info:

This amp follows the Hoffman AC30 schematic exactly. I ended up doing a single channel top boost, so no normal channel at all. It didn’t sound good enough to justify keeping in there.

I used a PTP layout on terminal strips which I think is pretty spot on. Here are the current issues:

Idle hum: I increased the capacitance of the first filter cap to 47uf and overall the amp became a lot quieter. There was about 6v AC on the positive leg of the cap and now it’s down to 1.5. I think the bias is a bit too hot in the output section and is the source of the hum. I’m using a 50 ohm cathode resistor with 310v on the screens and 320v on the plates and a +10v bias voltage on the cathode. Should this voltage be negative? The tubes aren’t red plating or anything but they are running very hot. Would I be better off with an 85 ohm 10 watt?

Wonky preamp: I’m getting a considerable amount of noise and hiss from the tone controls. The grounding scheme I used for the preamp is what’s suggested on the layout with the bus soldered to the pots. Even with the input jack switched to ground, lots of hiss is bleeding through, although I’ve double checked that my grounding scheme is correct and good, I could use some suggestions on alternatives or other ways to confirm the grounding working 100%. With the volume all the way down I have signal bleeding through when the treble control is turned up. If anyone has suggestions for why that might be, I could use some direction for that. I will attach images of the build, I changed some of my layout decisions and all solder joints are confirmed good. All problems go beyond that.

I need to post from an image posting site since the forum has been giving me problems uploading pics. Thanks everyone!!

[Opry Audio]

https://ibb.co/2Kpq3sv
https://ibb.co/nLVnQvw
https://ibb.co/q5G4gsJ
https://ibb.co/pv1HzD1
https://ibb.co/KqZQM1w

[vampwizzard]

Opry, can you show us a birdseye photo of the whole layout? Can you chop stick the amp, moving signal wires away from power sections? Did you use shielded wire anywhere? 

[Opry Audio]

I totally forgot to get a good full gut shot. I will post that as soon as I’m back on the bench. Moving signal wires has no effect. I didn’t use any shielded wire but I’m thinking that may be essential in this layout

[shooter]

Should this voltage be negative?

It is, you just gotta stand on your head, or read this;

http://www.aikenamps.com/index.php/what-is-biasing

[Opry Audio]

Should this voltage be negative?

It is, you just gotta stand on your head, or read this;

http://www.aikenamps.com/index.php/what-is-biasing

Thank you! This answers my biasing question completely.

[Opry Audio]

Do you think I would have better luck taming the bias a bit by increasing R28 and R29 to 470k?

[sluckey]

R28 and R29 will have no effect on bias. R34 will.

[Opry Audio]

R28 and R29 will have no effect on bias. R34 will.

I may bump R34 up to 85 ohms.

What’s the purpose of R28, 29 exactly?

[shooter]

here's more fun with self biasing

"CALC Rk cathode bias

Load impedance, divided by Mu(g2), then times 0.6, should be a fair trial cathode resistor.

This may melt or freeze. Adjust B+ or Rl until in ballpark.

Since Mu(g2) is "10" for 6V6, 6L6(GC), EL34, and most large audio-market bottles generally, once you nail your OT, then you know your rough Rk (but have to compute safe B+). EL84 Mu(g2) is 18, so it will be different

Once you pick an Rk here’s the Idle DC math

I = E/R so measure the VDC at cathode (E) / by Rk = total series current

Plate VDC – Vk * calculated I (above) = Pdiss (basically)
 
Compare Pdiss you just mathed against the Pdiss for your tube from the datasheet in the section applicable to your config. A1 SE, PP AB2………."

[Opry Audio]

So according to that math the total cathode current is 200mA and the plate dissipation is 17w.. way too high, yeah? . An 85R will bring it down to 117mA with 71.1% of maximum rated dissipation for an el84. That is something I will absolutely do. Maybe a lot of the idle hum will go down to a reasonable level once the tubes are biased properly.

[Opry Audio]

After increasing Rk to 108 ohm (220 5w in parallel is what I had on hand) my hum problems have not improved. Here’s the voltages for the output stage. Spot something that doesn’t look right?

Screens - 360v
Plates - 360v
Cathode - 13v
Rk - 108

Is the PV too high? EL84 datasheet says 300v max but I know plenty of amps run up to a 400v PV but not sure if it’s right for this output config

[Opry Audio]

According to math it should be 82% plate dissipation. I’m getting some nasty pops and clicks, but maybe bias isn’t my problem. Just wanted to update!

[Opry Audio]

https://ibb.co/MsNmcPP
https://ibb.co/SQ9LDJt
https://ibb.co/YdSPMMn
https://ibb.co/pb8YQBt

Here are some full gut shots as requested and some pics of the grounding for the preamp

[Opry Audio]

And here’s a video so you can see for yourself

https://vimeo.com/365852804

[tubeswell]

According to math it should be 82% plate dissipation. I’m getting some nasty pops and clicks, but maybe bias isn’t my problem. Just wanted to update!

Pops and clicks can just be a bad/cold solder joint. Wriggling each component lead with a pair of needle nose pliers can help find these. If the pops and clicks are new, look carefully around the area(s) you recently worked on.


That hum sounds like 60Hz (to my ears), so could be from the heaters or interference from the lighting in your room (does it hum equally in the dark?)


For interference hum, you could try shielded cable to the grids of the pre-amp stages. Grid stopper resistors on the pre-amp stages will also help a bit.


For heater hum, a bit of heater elevation can help. Also, check the phasing of your output tube heater pins. You ideally want them wired so that any heater hum from that source is cancelled out in opposite halves of the OT primary. (This means keeping all pins wired in parallel. Count how many twists you gave to the heater pairs between each output tube)


I note your pre-amp ground returns appear to be scattered along the buss bar. I suggest reading Merlin Blencowe's article on galactic grounding layouts. Also at your signal ground point - did you scrape the metal coating on the chassis back to bare metal for a good electrical contact?

[Opry Audio]

I note your pre-amp ground returns appear to be scattered along the buss bar. I suggest reading Merlin Blencowe's article on galactic grounding layouts. Also at your signal ground point - did you scrape the metal coating on the chassis back to bare metal for a good electrical contact?

Yep! Although I don’t know if grounding is my problem. This amp is in much better shape than when I originally posted. After making some layout changes, there’s no more idle hum.. my power tube cathode to ground line was too close to the screen grid voltage.

I have just about one or two things left to address before calling it done. There’s a lot of hum and hiss when turning up the volume control. Having it completely down kills it, grounding the input grid of the CF kills it, so it must be before the volume control. I’ve tried all kinds of different tests to get rid of it for good but this bugger just won’t quit. Everything else sounds great, I just would love for this amp to be quiet as a mouse so the top boosty goodness can be heard.

The only changes I’ve made from the Hoffman schematic is 470k grid leaks after the phase inverted instead of 220k, and I took out the normal channel. The unused triode for that input stage is grounded.

I’ve pretty much ruled out the heater supply, there’s no DC on the CF input grid, I’m using shielded cable wherever there was verge of oscillation, but no luck getting rid of this hum. It gets a lot worse with the input jack unswitched from ground. I tried moving the preamp filter cap ground to the ground bus close to the input stage but no difference. I have a feeling that this might be a PS issue, or a problem with the CF, maybe something before that. Whatever I’m hearing is being super amplified by the CF.

Pulling V1 gets rid of a lot of the hiss. Pulling V2 stops the hum completely (probably related to the input grid)

[tubeswell]

So you tried some different tubes in V1 and V2?

[Opry Audio]

So you tried some different tubes in V1 and V2?

First thing I tried 😭

[tubeswell]

Then I'm thinking the hum from V2 could be a bad plate resistor or bad solder connection in V2. (Some of your solder joints in the pictures look a little on the skimpy side - maybe re-flow some of them to get the solder covering the lug holes)


(Did you manage to eliminate the pops and clicks?)

[Opry Audio]

Then I'm thinking the hum from V2 could be a bad plate resistor in V2.


(Did you manage to eliminate the pops and clicks?)

 I already replaced the 100k on the plate of V1 for a metal film 1w should be plenty quiet. One thing I did notice is that the wire going from Ra to the plate sounds like it has AC on it.. like you can tap it and it sounds kind of like a microphonic tube would sound. I haven’t tried replacing the 100k on V2 though. I’ll give that a try.

I’ve only really gotten clicks and pops as the amp is warming up, and I think it might have something to do with whatever I’m experiencing on the plate of V1. But to be honest I’ve been poking around a lot and I can’t find any bad joints, been using lots of flux to get the good flow.

Just to be clear, I can hear the hum coming through when the switch is grounded, and then it’s super loud when the jack in unswitched from ground. It sounds conclusively like 60hz to me. I can see it really clearly on the 100k in the tonestack, so I need to backtrack to see where it begins. But as a point of reference it’s there. The treble control affects that frequency in a way where I can see oscillation riding on top of the frequency and then it oscillates when the treble control is turned to 10.

[Opry Audio]

Like I mentioned before, the hum is totally killed with the input grid of the CF sent to ground

[Opry Audio]

(Some of your solder joints in the pictures look a little on the skimpy side - maybe re-flow some of them to get the solder covering the lug holes

I didn’t see this right away, but that could be true, I will reflow and report back. There’s potential for a 60hz hum to be produced from a bad solder joint in V2?

[Willabe]

The only changes I’ve made from the Hoffman schematic is 470k grid leaks after the phase inverted instead of 220k,...

Can't do that, it will burn up the EL84's.

Put the 220K's back on the EL84 grids to ground. 

[sluckey]

The only changes I’ve made from the Hoffman schematic is 470k grid leaks after the phase inverted instead of 220k,...

Can't do that, it will burn up the EL84's.

Put the 220K's back on the EL84 grids to ground.

Not so. I have several EL84 amps with 470K grid resistors. The Hammond AO-39 also uses 470K grid resistors.

[Opry Audio]

The only changes I’ve made from the Hoffman schematic is 470k grid leaks after the phase inverted instead of 220k,...

Can't do that, it will burn up the EL84's.

Put the 220K's back on the EL84 grids to ground.

Not so. I have several EL84 amps with 470K grid resistors. The Hammond AO-39 also uses 470K grid resistors.

The only reason I did it is because I spotted it in an el84 Mesa amp and it was in a ceriatone schematic. Truth be told I don’t know fully what they are doing, so if anyone would like to offer that info I’d be grateful. It made the EL84’s run much cooler, and it did help bring down the noise floor. Not sure if it’s better or worse, just an objective result that I got

[Opry Audio]

https://el34world.com/Forum/index.php?topic=23576.0

My q got answered in this thread for anyone reading this in the future who has the same q

[Willabe]

The only changes I’ve made from the Hoffman schematic is 470k grid leaks after the phase inverted instead of 220k,...

Can't do that, it will burn up the EL84's.

Put the 220K's back on the EL84 grids to ground.

Not so. I have several EL84 amps with 470K grid resistors. The Hammond AO-39 also uses 470K grid resistors.

I stand corrected.

(But, isn't an AC30 running the EL34's hotter than a Hammond AO-39?) 

[sluckey]

Sure, but it's because of higher voltages and that 50Ω cathode resistor (for 4 tubes), not the grid resistor. RCA tube manual specifies 1MΩ for the max grid resistance for cathode biased circuits for 6BQ5.

[Willabe]

Yeah, I looked it up after your post and saw that.

 

 

[Opry Audio]

Then I'm thinking the hum from V2 could be a bad plate resistor or bad solder connection in V2. (Some of your solder joints in the pictures look a little on the skimpy side - maybe re-flow some of them to get the solder covering the lug holes)

No luck with reflowing. I think the problem might go as far back as the input jack. I feel like I’m losing perspective though. When taking an AC or DC reading on the tip of the input jack I’m getting High frequency oscillation and at the grid of V1, low frequency oscillation, kinda like motor boating.

[Opry Audio]

And! I’ve been doing a lot of these tests with the input jack switched to ground, but now that I have something plugged in, grounding the input grid mostly kills the hum. I can still here like a small amount coming through the other stages. That signifies that it must be my input jack wiring or input jack grounding?.. or maybe a ground loop

[tubeswell]

And! I’ve been doing a lot of these tests with the input jack switched to ground, but now that I have something plugged in, grounding the input grid mostly kills the hum. I can still here like a small amount coming through the other stages. That signifies that it must be my input jack wiring or input jack grounding?.. or maybe a ground loop

Or a dodgy/intermittent input jack tip switch contact that's not closing properly

[Opry Audio]

And! I’ve been doing a lot of these tests with the input jack switched to ground, but now that I have something plugged in, grounding the input grid mostly kills the hum. I can still here like a small amount coming through the other stages. That signifies that it must be my input jack wiring or input jack grounding?.. or maybe a ground loop

Or a dodgy/intermittent input jack tip switch contact that's not closing properly

I want it to be this but I just don't think it is 

I'm pretty convinced this is for sure a ground loop. I just read on another thread about using the input jack as the one and only connection to chassis. I followed Doug's grounding scheme for this one with a bus wire on the back of the pots and my PS and mains ground at another chassis point. Is it kosher to run the PS star ground from the first filter cap through the preamp bus and ground through the input jack? and keeping the mains ground over where it belongs of course.

[tubeswell]

And! I’ve been doing a lot of these tests with the input jack switched to ground, but now that I have something plugged in, grounding the input grid mostly kills the hum. I can still here like a small amount coming through the other stages. That signifies that it must be my input jack wiring or input jack grounding?.. or maybe a ground loop

Or a dodgy/intermittent input jack tip switch contact that's not closing properly

I want it to be this but I just don't think it is 

I'm pretty convinced this is for sure a ground loop. I just read on another thread about using the input jack as the one and only connection to chassis. I followed Doug's grounding scheme for this one with a bus wire on the back of the pots and my PS and mains ground at another chassis point. Is it kosher to run the PS star ground from the first filter cap through the preamp bus and ground through the input jack? and keeping the mains ground over where it belongs of course.

I would follow Merlin Blencowe's 'galactic' ground scheme if it was my build. I've used this is lots of amps and it works perfectly. YMMV

[Opry Audio]

And! I’ve been doing a lot of these tests with the input jack switched to ground, but now that I have something plugged in, grounding the input grid mostly kills the hum. I can still here like a small amount coming through the other stages. That signifies that it must be my input jack wiring or input jack grounding?.. or maybe a ground loop

Or a dodgy/intermittent input jack tip switch contact that's not closing properly

I want it to be this but I just don't think it is 

I'm pretty convinced this is for sure a ground loop. I just read on another thread about using the input jack as the one and only connection to chassis. I followed Doug's grounding scheme for this one with a bus wire on the back of the pots and my PS and mains ground at another chassis point. Is it kosher to run the PS star ground from the first filter cap through the preamp bus and ground through the input jack? and keeping the mains ground over where it belongs of course.

I would follow Merlin Blencowe's 'galactic' ground scheme if it was my build. I've used this is lots of amps and it works perfectly. YMMV

I wish I had read Merlin’s grounding chapter before I started this 😭 so many grounding regrets

[Opry Audio]

Just wanted to update and say this amp is totally complete. My 60hz hum problem was coming from the PS ground. I separated the reservoir from the preamp caps and the hum was gone completely! Thanks for your input y’all, onto the next build.

[Willabe]

Glad you got it fixed. 

Thanks for telling us how you fixed it.