Fired up the Classic 30->36W, it works, some issues?

[krash]

I had 125 ohm cathode resistors in the output stage and when I fired it up all the voltages looked pretty good (332V plates on 110VAC supply, with a variac), and the normal channel was working but not a lot of crunch. I put dodgy tubes in for the first fire-up so not sure whether bad tubes were to blame. After running it for a little bit I noticed that the well-worn Sovtek EL84s were looking a little too orange in the plate for my taste so I turned it off and then swapped the cathode res for 150 ohm.

When I fired it back up, it basically didn't have much sound but when I played hard with it cranked, I'd get intermittent tone with crackly sound, cutting in only when I play loud, then it quit altogether. No red plates but when I checked the voltage, the B+ was 374V. Could a bad set of output tubes cause this? Otherwise what would cause the B+ to go up 40V at the same time the amp basically quits making sound?

The PT has no center tap, this is bridge-rectifier into 32/32 caps with a 300V PT secondary (measured at 110V).

So did I likely kill the tubes?

Some other voltages are askew, mostly one of the preamp tubes' plate voltages is way high (like 275V) and the plate voltages on V1 are way low (like 125V). I must have smoked something. Could a bad tube or three lead to all of these screwy voltages? Before I put in another set I'd like some indication I am not going to smoke them... my other tubes are barely-used NOS.

[krash]

Alright here are the voltages with the variac set to 120V

With no power tubes (only preamp tubes) in the amp, the B+ is 407 V

Once I put the power tubes in, all the voltages are screwy. Most are low (linearly along with the B+) and some are high.

The rest of the readings are with power tubes installed and heated up:

V4. V5, V6, V7 pin 7: 280V
V4. V5, V6, V7 pin 9: 277V
V4, V5, V6, V7 pin 3: 35V (!!)

V3
---
pin 6: 163V
pin 1: 174V
pin 8: 62V
pin 7: 30V
pin 2: 30V

V2
---
pin 8: 135V
pin 6: 229V

V1
---
pin 6: 131V
pin 1: 126V


I didn't probe the other pins, since I figured job #1 is debug these voltages.

The amp is missing the bass pot (wires tied off that would go to it), and is missing the TMB channel master volume pot and instead I have put in a 220K resistor to ground at the PI input where the MV pot wiper would go.

Also other deviations from Richie's latest include point "A" on the layout drawing (wiper of normal channel vol) is connected to the 0.1uF cap on the PI rather than connecting that cap to the neg-feedback switch, and I have eliminated negative feedback. I have 150 ohm cathode resistors in the power stage but still have 100 ohm screen resistors. Of course this amp has a bridge rectifier and I didn't use a cap can (used regular capacitors for the main power inlet).

At the moment, it makes no sound. If I put my meter on "pulse" and put a signal on the PI input .022uF cap I can hear a buzz, but I don't hear anything from the 0.1uF side.

I figure some of my problems are normal debug and may be due to a couple of my deviations from the standard design. But the 35V cathodes (3x the expected value) and the low B+ voltages have got me kind of worried. The PT does buzz mechanically more than I had hoped it would.

The first time I fired it up at 110V from the variac, I was reading 332V on the plates. I think maybe I damaged the tubes? I did change one pair of them before doing this latest set of measurements.

Any help out there? You can see the layout here:

http://prophetsandpoets.com/josh/images ... ready_.jpg
http://prophetsandpoets.com/josh/images ... stack_.jpg
http://prophetsandpoets.com/josh/images ... al_ch_.jpg

[krash]

Alright ... well I put the old Valvo (barely-used, NOS) tubes in the amp and fired it up ... same thing. 280ishV plate, more like 38V cathode, faint orange glow in the plates so I am guessing I have now killed a total of 10 tubes in it.

Also guessing the redplating has caused the tubes to fail short and that's why I'm gettng high cathode readings and low B+.

So should I spring for a quad of JJs and give it a go or chew up more of my NOS tubes? I have some 7189's kicking around.

How can I check this out without tubes and verify it's not going to die? I've measured all the R's and checked the grounds and it looks real good, 407V unloaded B+ is perfectly in the ball park.

[krash]

I have 150 ohm resistors for the cathodes, one resistor per tube pair. 100 ohm on the screens. No choke or resistor ... maybe that's the problem? Where does this choke or resistor go?

[Plexi]

between the stages of the filter cap.. the high side b+ is the OT centertap connection..the other side of rhe resistor is where the supply to the board and screens goes..

Richie

[krash]

I am missing that resistor!

Hopefully as a result I didn't kill the PT.

[krash]

OK I put in the 1K res and I think maybe all my tubes are dead. When I first fire it up B+ is about 375V and then begins to drop as the tubes heat up. The cathode voltage on the power tubes rises above 11V as they heat up and I'm sure they'll go back up to 35V or so before all is said and done.

What could be causing this other than bad tubes? What else should I check?

[Ballz]

You may have DC leakage on the power tube grids, or faulty screeen resistors.

Check connections, double-check. I am sure you have made an error someplace when you swapped the cathode resistors.

Cheers /R

[krash]

Thanks. I have indeed checked, double-checked, re-checked, and gone over every component with a fine-toothed comb. I think it may be bad tubes making it impossible for me to get any measurement with it up and running and I'm out of good tubes at the moment. I am reticent to put these maybe-bad tubes in my other 18W to test in case they are capable of causing more damage.