Bassman Project Problem

[Subjecttochange]

Oh... did you try fixed vs cathode? Any comments on that? I have an extra 250R power resistor laying around I could use as a cathode- if it's not too low of a value.

[guitardude57]

Shared Cathode S/B 270-350 ohms. Double that for individual R's.

I have a real oddball amp going on myself. Old Pioneer tube amp gutted and rebuilt a few times, trying different amp circuits. It is a TW now.

I can run either 7189 tubes or 6L6's. Take out one set, and put in the others and go...........I like it.....no Biasing. Each tube pair have their own CR's. And both types of tubes have no issues with 467V plate and 410 screen. 7189 is a higher spec EL84 type tube. Very chimey. So a flexible setup.

Since the PT has 2 different heater supplies, the 1.5 amp feeds the 12ax7's and the 7189's. The 3 amp one feeds the octals......6L6's.

I should try EL34's.......

I have a 3500R 50 watt between the plates(doubler circuit.......old tranny...)
and the screens. I will eventually change that to get the voltages a little closer than the 50 volt difference now. It is plenty loud now and like I said....no problems after 30 hours of operation.

[Subjecttochange]

Well that's encouraging. Would the 7189s work in this amp you think? I had an express that I loved- but I always wanted to tweak that circuit. Wasn't quite right for me- but it was close.

This is getting to be stranger and stranger.

Ok... so I went back and re-soldered all of the could be problem points. Fired on the mains (no tubes)... no reading on Pin#5. Before I was getting something small but fluctuating. I figured that was good.

Fired on HV... and the 4.7K resistor immediately fried. What to do? It's only a 1/2W. I think I am going to grab a 2W tomorrow at Radioshack or Fry's.

I didn't get a chance to look at the voltage... hmm wonder why.

[Bob-I]

Hmm... you're problem is obviously the bias circuit. Can you post close up pictures and the schematic? Check the polarity of the cap on the bias supply, remember that this is a - voltage so the cap goes + to ground. Check the AC voltage into the bias supply, and the DC voltage coming out of the bias supply, then into the 220K bias balance resistors and again at the grids.

I wouldn't power up the high voltage until you have a good steady -40-50 VDC on the grids, pin 5.

[mlp-mx6]

Dude, you need to step back and recheck your wiring. DO NOT just sub a 2W resistor for a 1/2W resistor and expect that things will be OK. Once you are as certain as you can be that the wiring is correct, ENSURE that the voltages (with no tubes) are where you expect them. Ideally, disconnect all your HV connections to the board and put them back one-at-a-time. Proceed ONLY when you are sure each voltage is as it should be (given that there are no tubes allowing the current to actually flow).

[Subjecttochange]

Well truth be told... it is supposed to be a 1W resistor and I accidentally used a 1/2W (omission upon transcription of schem. error). So that needed to be redone anyway.

But yeah... I am going to re-work/re-do the bias circuit today to see if I can figure it out. Why/How would it be that the amp would work with the other rectifier setup though? That's what gets me...

I kind of pieced together some schematics- I'll try and draw one up sometime today for the power section.

Thanks a bunch you guys have been super helpful. Still trying to learn the ropes.

[Structo]

I know it might seem obvious but have you checked the bias pot to make sure it is good?

Should be a linear pot.

Probably have to take it out of circuit and put a meter on the outside lug and the other lead to the middle wiper lug.
Rotate the pot slowly and watch the ohm reading.
It should steadily go up or down depending how you have the meter connected.

If the reading jumps around or has dead spots, the pot is bad.

[Subjecttochange]

Alright... went to the store- got a few parts.

Came back and resoldered all of the bias circuit joints. A few were questionable. Checked the Bias Pot as per Structo's request.

Bias pot is ok... unfortunately. I was hoping for an easy fix.

The grid is still off- but on the right track. When I flip on mains the grids are at ~-19V. Haven't flipped in the HV- but I did replace the 4.7k.

I am going to re-wire the bias circuit- rather than just redo the joints. See if that helps.

[Subjecttochange]

Same scenario when it was redone... still about -19V.

My bias circuit is exactly the same as the circuit on the ceriatone layout.

Tried the HV- no problems with the new resistor. They didn't have 1W or 2W 4.7k- just a 5W... so if there's a problem anywhere- that will be the last place it manifests itself.

I think my next idea is to just bypass the pot and use a 56k resistor in place of the 25k pot+27k resistor- as seen on the weber schematic

https://taweber.powweb.com/store/5f6a_schem.jpg

[Jana]

+1 what Bob-I and others have said. It's in your bias. -19volts for bias is going to run 6L6 tubes way hot, redplate hot, meltdown hot!

You should be seeing about -50 volts there.

[Subjecttochange]

So what should I do? I have re-wired the circuit 3 times. So either I have missed something every time or something else is wrong?

Should I change the resistor?

I am currently getting -72V behind the diode- but -19V everywhere else. I replaced all the resistors and have removed the pot so currently it's non-adjustable fixed bias. Should I replace the caps? I hope they're not bad. They're spragues... I actually spent money on these.

I'll try that though. Be back in a little...

[mlp-mx6]

Post some pictures so we can see.

Is the bias supply cap oriented correctly? (+ to ground)

[Jana]

Blindly replacing parts isn't the way to go. Find out why it is doing what it is. Don't assume anything.

Do you have the bias filter caps in the right way? (polarity wise).

-72 volts behind the diode? Is that on the AC side or the rectified side? Which end is "behind?"

What is the voltage drop across the 15K resistor?

Double check the values of all your parts. Do you really have the values you think you have in there? Mistaking a red band for an orange band on the multiplier can make a huge difference.

Draw up a schematic of exactly what you have for the bias circuit, the complete circuit with AC and DC voltages at all points.

Post it for us to see.

[Subjecttochange]

Currently this is what it looks like.

The voltages sound much better to me. Now I just need to take out the 56K and replace it with the 27k and 25kL pot

Regarding colors... I am colorblind- so I check everything with the DVM before putting it in. Kind of slow going that way- but it forces me to be organized.

[Subjecttochange]

I just love MS paint. haha. It works I guess.

All the other voltages check out fine. Sounds great- little quicker on the attack too- as expected.

Thanks again guys. Do any of you know how to do switchable cathode/fixed bias? Is it pretty intensive. I would love to work that in esp. for a bit more of that character out of the EF86 channel.