Farfisa FR40 Restoration - Bias Problem

[whoops]

Hello,
I'm restoring at the moment a Farfisa FR40 amplifier.
It uses 2x EL34 tubes for the output stage.

I ordered a matched pair of EL34 and I'm now measuring and setting the Bias.
B+ voltage is 323V

This is the schematic of this amplifier:
http://b.baldach.free.fr/farfisaFR40/FarfisaFR40_Schematic.jpg


PROBLEMS:
The bias current seems a bit high it's around 65mA, but thats not my main concern at the moment.
After some minutes of the amplifier warming up one tube starts to draw much more current than the other one. It goes up slowly, after some minutes one is drawing 65mA while the other is 73mA, but after a while it rises to 80mA up until 104mA, the amplifier starts to output some noise from the speaker (nothing connected to the input) when the second tube goes draws more than 80mA.

I swapped the tubes one for the other and the problem stayed in the same socket and did not follow the tube, so I guess the problem is in the circuit and not in the tubes themselves.
 


I thought the problem might be DC leakage from the 47nf capacitors, so I replaced those,
I also measured a 20K diference from one of the 220K resistor compared to the other so I replaced both of those.

The problem still persists after those changes.
Any idea of what might be the problem? what do you think I should do next?

Thank you so much for your help

[shooter]

After some minutes of the amplifier warming up

pull the PA tubes, bring it up and monitor the grid bias at the tube socket, does it drift?
are you using  1ohmers?
another thing to try, might squeal, swap OT primary leads and see if your problem "moves" with the leads, could be a parial short, or enamel breaking down with heat 

[whoops]

pull the PA tubes, bring it up and monitor the grid bias at the tube socket, does it drift?

Will try that, thanks

are you using  1ohmers?

I'm measuring Bias with this unit, the probes have 1ohm resistors installed

another thing to try, might squeal, swap OT primary leads and see if your problem "moves" with the leads, could be a parial short, or enamel breaking down with heat 

Will try that out also, thank you so much

[sluckey]

Monitor the voltage on pin 5 of the socket with the high current. What happens to that voltage when the tube current starts rising?

[whoops]

Thank you so much Shooter and Sluckey for your advices,
I will proceed as you suggested:


1) Pull Power Tubes out, bring volume up and monitor grid bias at the tube socket - check if it drips
2) swap the OP transformer primary wires to see if the problem moves with the leads
3) Monitor the voltage at Pin5 of the Power tubes - see what happens to that voltage when the bias drift rises


I will do these tests next week, if there any other tests or suggestions please let me know and I will try it also
thank you so much for all your help

[whoops]

I'm testing at the moment the amplifier,
here are the results of the tests:

1) Pull Power Tubes out, bring volume up and monitor grid bias at the tube socket - check if it drips

Without the power tubes

B+ 453V
PIN5  tube A (the side that drifts) -21,7V
PIN5  tube B -21,7V

Stable voltage readings no drift occurs with time or after warm up

_______

2) swap the OP transformer primary wires to see if the problem moves with the leads


Swapped the OP transformer primary wires, got the big "Squeal" , had to reverse the input grid wire between the tubes (pin5) to remove the squeal and make the readings

Measured the Bias values, the drift occurs but didn't follow the OT transformer wire it stayed at the same socket
___________

3) Monitor the voltage at Pin5 of the Power tubes - see what happens to that voltage when the bias drift rises

with the Power Tubes installed here is what I measured over time:


_____________

It seems the problem might be some imbalance in the circuit but not in the Output transformer,
what do you think?

Any advices for what to look next?

Thank you so much

[shooter]

guessing you have 2 bias probes?
If so, swap them and repeat test 3
otherwise, still scratching head since swapping the grids solves creep from PI.

[whoops]

guessing you have 2 bias probes?
If so, swap them and repeat test 3
otherwise, still scratching head since swapping the grids solves creep from PI.

It was a long day working in this amp...
I swapped the bias probes, and the Drift still happened in the same socket.

I then proceeded to replace the old power tube's sockets for new Micalex ones.
While I was at it I inspected all the components, I found an 2watt 4K7 resistor that looked burned, so I replaced that.
I measured it after removal and it still measured fine, but maybe it drifts value after getting hot.



I also replaced some shielded wires, has the rubber in the old shielded wires used in the amplifier 50 years ago was disintegrating.
The normal, one conductor, wires all look good.

I replaced also the 2x 2K2 resistors, the 2x 1K resistors.
The parts market with balloons are the parts that were replaced until now. With Green it's the Electrolytic capacitors that were all replaced 2 months ago when the amp started to receive service.



Phase Inverter is a new Electro Harmonix, it's matched, checked it with the Orange tube tester.
El34 are new Tung Sol, they are a matched pair and I tested them with the Orange tube tester also, one gave the value of 8 and the other a value of 9.

After checking everything, we plugged the amp, the Bias readings seem stable and really close to each other between the 2 tubes.
For a moment we thought the problem might have been solved.
Then we had to turn the amp off to reverse the reverb tank wires (in and out leads were swapped).

Turned the amp on again and the Drift in the Bias reading starts again, in the same tube socket.
I'm really tired, it was a long day and I'm a bit frustrated, have to think about it again after rest.

The reverb was not working (before I received the amp) and it's still not working, I don't know if it has anything to do with this problem (Bias Drift) or not, maybe it was a coincidence that we swapped the reverb tank in/ou leads and the Bias Drift decided to appear again.

Thanks for your help and for your patience.

[shooter]

I would pull all tubes except PI and power
verify drift again
did you change all the Ecaps in the PS, including the ones for the bias supply?

[sluckey]

I'm thinking your tubes are biased way too hot. Adjust the bias voltage at pin 5 to -35VDC. Then use your bias probes to check bias current . Fill in a table just like the previous table with the new numbers. Post the new table. Any drift?

[shooter]

I'm thinking

    I never done the math 
23W seems high, looked up my 34 #'s and notes say 19w idle 

[whoops]

did you change all the Ecaps in the PS, including the ones for the bias supply?

yes, all electrolytic caps in the amplifier were replaced some months ago

[whoops]

I would pull all tubes except PI and power verify drift again

Will do that , thanks

[whoops]

I'm thinking your tubes are biased way too hot. Adjust the bias voltage at pin 5 to -35VDC.

yes, the present Bias is really too hot.
I would like to modify the circuit to be able to adjust the bias with a Pot.
What do you advise to make the bias adjustable?

I could then adjust it to -35VDC and test again

Thank you

[shooter]

I'm over my pay-grade here so;
curious if my snippet is correct on the bias supply?  what I'm seeing is a voltage divider with 2 series filaments and a 47 ohm R getting me -25vdc.

If I guessed correct, then I don't see a lot of adjustability in the Bias supply 

thanks

[whoops]

I'm over my pay-grade here so;
curious if my snippet is correct on the bias supply?  what I'm seeing is a voltage divider with 2 series filaments and a 47 ohm R getting me -25vdc.

If I guessed correct, then I don't see a lot of adjustability in the Bias supply 

thanks

The resistor is 47K, although in the schematic it seems it's an R and not a K. If you check the resistor values in the scheme all the "R" resistors dont have a letter next to the number, only the "K" and "M" range have the letter.

It was suggested to me to replace the 47K resistor, with a 22K resistor in series with an 50K pot to be able to adjust the bias going into the Power Tubes. But I was not seeing the P3 and S3 connection (P points and S points are the connections between Power amp and Preamp).

I don't know what those Y1 4 5 and Y2 4 5 are , I have to check it in the preamp section.

[PRR]

> I don't know what those Y1 4 5 and Y2 4 5 are

It's "V" for Vacuum tube. Preamp tube heaters. So you are kinda stuck with ~~25V there.

[sluckey]

That's a very poor design on the bias supply. I've never seen an EL34 circuit that only had -20v bias. That's why your tubes are biased too hot and not stable. The EL34 bias circuits I've seen typically run between -30V and -45V. Maybe tubes were tougher when that amp was new. Have you replaced all three bias caps? Try this... Pull V1 and V2 to remove the filament load from the bias supply. What does the bias voltage on pin 5 of each EL34 read now?

I would consider modifying the bias circuit to provide a proper bias voltage to the EL34s. Leave the supply as is to power V! and V2 filaments and build a separate supply for the bias. I would build the bias supply seen in this schematic. The bias range resistor is probably the only change needed for EL34s. Here's a link to the schematic...

     http://sluckeyamps.com/phoenix/phoenix.pdf

[shooter]

I was wondering if this might be a good candidate for ~~100ohm cathode R's.  Since it's already an iffy design?  easy to try

[sluckey]

That's probably the easiest solution. My Magnatone M10A is fixed bias and also has a shared 100Ω cathode resistor. Definitely worth a try.

[whoops]

Thanks you so much for your insight,
is there any particular reason to have different Heater supply for the 2x 12AX7 tubes?

They are wired in series, are they receiving 25VDC?

Could the Heater supply for the two 12AX7 tubes be re-wired so the filaments are in parallel and supplied by the existing 6,3V AC supply that's used for the other tubes? (S2 and S7)

[whoops]

I was wondering if this might be a good candidate for ~~100ohm cathode R's.  Since it's already an iffy design?  easy to try

I'm sorry, I'm not aware of that type of circuit, can you show an example?

[sluckey]

Before you do anything to V1 and V2 filaments, pull the tubes as I suggested. If the bias voltage on pin 5 of the EL34s doesn't increase to approx. -35V then there's no need to rewire the filaments.

Shooter's suggestion is to simply connect a 100Ω resistor between pin 8 and ground for each EL34. Of course you need to remove the existing ground connection. Look at the Magnatone M10A schematic to see an example.

[shooter]

is there any particular reason

someone thought so, I suspect it's lost along with the scrolls from the library at Alexandria 

[whoops]

Have you replaced all three bias caps?

Yes, all Electrolytic capacitors in the amp/schematic were replaced already. Power amp, Preamp and Power Supply

[whoops]

is there any particular reason

someone thought so, I suspect it's lost along with the scrolls from the library at Alexandria 

Really strange,
there's already an Heater supply in circuit, that is supplying all the tubes in parallel, it would make sense to supply the 2x 12AX7 preamp tubes in the same manner. I don't see why they choose a diferente Heater supply just for these 2 tubes.


Maybe the stock Heater tap on the transformer was not powerful enough to supply all the tubes power consumption needs?

[whoops]

Before you do anything to V1 and V2 filaments, pull the tubes as I suggested. If the bias voltage on pin 5 of the EL34s doesn't increase to approx. -35V then there's no need to rewire the filaments.

Shooter's suggestion is to simply connect a 100Ω resistor between pin 8 and ground for each EL34. Of course you need to remove the existing ground connection. Look at the Magnatone M10A schematic to see an example.

I will remove V1 and V2 and make the measurements as suggested, thank you

I will check the Magnatone M10A schematic,
what is the goal of having the 100r from pin8 to ground? how does that affect the Bias voltage and circuit?

thank you so much Sluckey, Shooter and PRR, your help is invaluable

[sluckey]

Not strange at all. Lot's of amps out there that have dc filaments on the preamp tubes. Less noise than AC heated preamp tubes.

What is strange is that they are sharing the bias supply in such a way that you can never have over -25v for the actual bias circuit.

Have you pulled V1 and V2 and checked the bias voltage at pin 5 of the EL34s?

[whoops]

Have you pulled V1 and V2 and checked the bias voltage at pin 5 of the EL34s?

Unfortunately I can't go to the shop today, I had gigs yesterday, have have 2 more over the weekend.
I will do that test along any others you advise next week.

Thank you Sluckey

[whoops]

Here is the Magnatone 10A Bias circuit, I'm trying to understand it

[shooter]

when you're NOT in standby (amp plays music) the 100 ohm R becomes part of the overall bias.  you start with the (fixed bias) -17 (your case -25) then add (away from 0) the vdc at the top (cathode) side (self bias)  100 ohm R, making you bias more negative - cooler running tubes

[whoops]

when you're NOT in standby (amp plays music) the 100 ohm R becomes part of the overall bias.  you start with the (fixed bias) -17 (your case -25) then add (away from 0) the vdc at the top (cathode) side (self bias)  100 ohm R, making you bias more negative - cooler running tubes

Oh I see, Cathode Bias , some people call it self bias. Thanks for bringing that up I heard about it but never worked in a amp with Cathode biasing
People seem to prefer or use that type of bias in under 30 watts amps, or class A amps. Like the Magnatone amp. Cathode Bias changes the way the amp responds also.

Thanks for bringing that up, as it is probably the most simple and easy solution, but I prefer to keep this amplifier with Fixed Bias, and have an adjustment Pot.
 

I would consider modifying the bias circuit to provide a proper bias voltage to the EL34s. Leave the supply as is to power V! and V2 filaments and build a separate supply for the bias. I would build the bias supply seen in this schematic. The bias range resistor is probably the only change needed for EL34s. Here's a link to the schematic...

     http://sluckeyamps.com/phoenix/phoenix.pdf

I prefer going this route, leave the supply to the V1 and V2 filaments and AC151 transistor as stock, and I can easily add a small transformer just for the bias supply doing the circuit Sluckey suggested.

Another thing that could work, and this was done by user "Racing" in a thread I recently found, he used a voltage doubler circuit for the bias.
" it turned out that bias voltage,PT carries separate taps for that,was way to low.
No WAY i was gonna get the voltage needed to control this thing with the ideas i had i mind.Well. Voltage doubler time.
See..them taps ALSO runs rectified heaters for two of the preamp tubes AND the voltage needed for the AC-151 transistor.A voltage doubler setup tho handled the whole thing flying colours. Got me 66VDC raw...which is ample enough."
http://www.offsetguitars.com/forums/viewtopic.php?f=10&t=90637

[sluckey]

I prefer going this route, leave the supply to the V1 and V2 filaments and AC151 transistor as stock, and I can easily add a small transformer just for the bias supply doing the circuit Sluckey suggested.

You don't need another transformer to do what I suggested.

[whoops]

You don't need another transformer to do what I suggested.

So how would you approach this?

[sluckey]

You don't need another transformer to do what I suggested.

So how would you approach this?

Verrry carefully.

[whoops]

You don't need another transformer to do what I suggested.

Oh I see, you would take the supply for the Bias circuit from the one half of the 300V tap.

Sorry Sluckey, when I saw the first time your Phoenix documents, I didn't see it was using the HT tap of the transformer
Thank you so much

So here it is applied to the FR40 schematic:

[sluckey]

That should work fine. Hope you have room inside the chassis for it.

[whoops]

That should work fine. Hope you have room inside the chassis for it.

yes the chassis is small, have to check the it could fit, I think it's a tight fit but doable.
thanks