5f2a bias voltage too high?

[MWJB]

Always measure volttages with all tubes installed, otherwise the lack of current draw will push the voltages up and give you a false reading.

With an ohmmeter, measure resistance from the grounded eyelet of the cathode resistor to the chassis, you want the meter to read the default lowest setting (maybe 0.3ohms or something like that?). If you have more than that, remake the ground connection. Did you use a brass plate or a buss wire?

Kendrick Roughneck uses a 470ohm cathode resistor.

[MWJB]

"so there will also be a little "cathode sag effect" while playing with the amp fully cranked IMHO." As long as the amp is cathode biased you will get the "cathode sag effect", the usual way to eliminate it would be to go fixed bias.

"or a 470 Ohms one as per your advice, even though I find it would set the tube bias a tad low ( but safer, as you wisely noted )." A 470ohm cathode resistor, with a 330-0-330VAC PT (with adequate current supply) & GZ34 will not set the bias "low". You should really get in the region of 60mA +/- a few mA.

My problem is that you keep plucking numbers (cathode voltages & currents) out of thin air, granted BD59s amp seems to have a problem, but we're better of dealing with the actual figures that he can report back to us, rather than magic-ing up numbers that we think should be there, which will just cause him more confusion. Let's see what he comes back with when he has a (measured & properly grounded) 470ohm cathode resistor and work from there.

[Robert M. Martinelli]

My problem is that you keep plucking numbers (cathode voltages & currents) out of thin air, granted BD59s amp seems to have a problem, but we're better of dealing with the actual figures that he can report back to us, rather than magic-ing up numbers that we think should be there, which will just cause him more confusion. Let's see what he comes back with when he has a (measured & properly grounded) 470ohm cathode resistor and work from there.

MWJB,

I'm sorry I caused you problems, but, believe me, I wasn't "plucking numbers out of thin air" to use your words, voltages on similar amps can be quite different, and I just used the figures b-d-59 gave initially, it's not my fault if later he gave us different readings.....

Anyway, since it was my intention neither to cause you problems nor to confuse b-d-59, I am going to unsubscribe, to allow b-d-59 to have his problem fixed more easily.

Accept my sincere apologies if I caused you and b-d-59 trouble.

Best regards

Bob

[MWJB]

Hi Bob,

Perhaps my use of the word "problem" was a little strong, "issue"? Is that better?

Don't unsubscribe. No need to apologise. You haven't caused me any problem.

I don't mean to make my posts sound like a personal attack on you, the idea is that they might be of benefit to BD59 and anyone else who has similar issues with an amp. If I see some advice, no matter how well intentioned, that I can't agree with, then I will speak up. The important thing is that the issues raised get solved, quickly & correctly. It's not about egos, personal slights & perceived offence. The facts speak for themselves and it's important to deal with the said facts and troubleshoot in a methodical manner.

All the best, Mark.

[big-daddy-59]

ok fellows I'm back, didn't mean to start any controversy.

For the sake of clarity,this is the schematic that I built from including changes:

[IMG]//s625.photobucket.com/albums/tt334/big-daddy-59/?action=view&current=Modified5f2a.jpg[/IMG]

I did use a 470R 5 watt power resistor for the power tube cathode bias and it didn't burn, just the circuit board underneath got a little scorched.

the 25u/25v cap on the other hand blew up, got a couple higher voltage replacements in the mail yesterday.

I have a suspicion that I need to order a different output transformer;one speced for a 15watt single ended amp with a primary impedance of 4k-5k.

I am going to reflow all the solder joints and put it back together with the 470R resistor, keeping it up off the board this time.

I really think that there might be something wrong with the tube, it's a cheap chinese one that I got primarily because I thought I might have difficulties with this being my first build

[big-daddy-59]

fixed link I hope

[MWJB]

Hi Big Daddy,

Some pics of the build, especially wiring around the 6L6 would be better than a schematic.

[tubeswell]

I have a suspicion that I need to order a different output transformer;one speced for a 15watt single ended amp with a primary impedance of 4k-5k.

Hi big-daddy-59

Excuse me not not going back to the start of the post - but has your OT got multiple secondary taps? (This would save you having to buy another OT). Or alternatively have you got a speaker lying around that is half the impedance of the speaker you have plugged in there at the mo'. (A 4R speaker into an 8R tap intended for an 8k load will produce a 4k reflected load).

As to the cathode resistor, on reflection I agree that 470R should be about right if the B+ isn't overly high (270 - what was I thinking? too many late nights). If you want the B+ a weeny bit higher, you can make that 1st reservoir cap 60uF in total (3 x 20uF in parallel) with a 5AR4 rectifier, depending on what you get at the plate. (You ought to get about 430VDC at idle by my reckoning - based on the info on your schematic). If your cathode voltage was about 33VDC (or so) with a 470R, that would be a dissipation of about 28W, which would be fine for a 6L6GC. If you want a bit better heat-handling for the cathode resistor, and you can't find a 10W 470R, you could put 2 x 5W 1k in parallel - for 500R 10W, although 5W should be fine for that ballpark of voltage at about that resistance. FWIW I stand the cathode resistor off the board a bit and make the leads stronger by twisting some additional solid core wire around each lead and flowing a bit of solder over it for strengthening.

[big-daddy-59]

first off I want to thank everybody for their help.....That said, after a long break where family life got in the way of my hobbies,I finally got my amp working. I finally traced the problem down...the input from the preamp was connected to the wrong output tube pin!! I had it connected to pin 6 when it's supposed to be connected to pin 5. Now everything is working properly ...390VDC at the plate, 30Vdc cathode, replaced the bypass cap with 50u/50v and upgraded the output transformer to a weber WSE15.

[limbe]

Hi Bob,

Perhaps my use of the word "problem" was a little strong, "issue"? Is that better?

Don't unsubscribe. No need to apologise. You haven't caused me any problem.

I don't mean to make my posts sound like a personal attack on you, the idea is that they might be of benefit to BD59 and anyone else who has similar issues with an amp. If I see some advice, no matter how well intentioned, that I can't agree with, then I will speak up. The important thing is that the issues raised get solved, quickly & correctly. It's not about egos, personal slights & perceived offence. The facts speak for themselves and it's important to deal with the said facts and troubleshoot in a methodical manner.

All the best, Mark.

I´ll second that! Limbe

[limbe]

I´m glad you got your amp going (no thanks to me) and hope you learned a few things in the process.Good luck in the future!

[Bruce / Mission Amps]

"with all tubes out got 360VAC from PT. With only rectifier in got the following":

With no tubes in it... there should be very little voltage drops across the B+ rail dropping resistors because there is no load.
So, V3 = 5Y3GT?
V2 = power tube?
V3 = 12AX7?

v3
pin 2 497VDC
pin 4 357VAC
pin 6 357VAC
pin 8 497VDC


V3 if the 5Y3GT, looks right to me, with no load.

v2
pin 3 497VDC
pin 4 476VDC
pin 6 -3.6
pin 8 0


With no power tube, you should have nearly the same voltages on lugs 3 and 4... since again, there is no load.
Lug 6 -3.6v? Where is that coming from? There is nothing on lug 6 of a 6V6 or 6L6. Something wrong there.

v1
pin 1 430VDC
pin 2 0
pin 3 0
pin 6 430VDC
pin 7 0
pin 8 0

Again NO TUBES INSTALLED.
430vdc to the plates of the 12AX7 socket?
Unless you have some really cheap filter caps, or they are just bad and they are leaking B+ to ground, or you have not mentioned some bleeder resistors somewhere in your B+ rail, you should have almost the same B+ here as you do on the screen node of the 6L6, lug 4. That's a 67vdc drop!! And with no tubes, that is across no load... so where is that 67vdc going?

71vdc at the cathode is simply outrageous and the first thing I would suspect is either a bad tube right from the start or a huge oscillation and you are measuring the combination of DC idle current with the additional current drawn by the tube in runaway.
If you don't have a wiring error somewhere.....
Another thing to suspect is that you actually have a 47 ohm resistor, not 470 ohms. But then I doubt you could get it to 71vdc.
With the proper B+ on the plate, you shouldn't need much more then a 510 @ 3 watt biasing resistor with a 6V6 in this amp and 510 @ 5w with a 6L6.
I have tons of 40 year old SE 6V6 amps here with 2 watt cathode biasing resistors.
A good running SE 6V6 with proper B+ levels, will have around 16- 20vdc at it's cathode, probably well under 20v in any condition.... the 6L6 can be 12-15vdc higher then a 6V6 but only if the PT can handle it and the single 6L6 tube is running in true Class A.