home brew 60watt 2X6L6. ht fuse and bias question.

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home brew 60watt 2X6L6. ht fuse and bias question.

Postby Anticpunk on Sat Mar 12, 2016 6:54 am

Hello all,

I recently built up an amp from an AB763 Twin Reverb schematic running a 1978 UL Twin Reverb output transformer and a repop earlier twin power tranny. I'm running 2 JJ 6L6GC in the output and pushing 8 or 16 ohms to match up the primary impedance. Biased to 36ma-ish per tube at around 440 on the plates. Measuring just under 60watts RMS into a dummy load with a 1Khz test tone. I fused the center tap on the hv/bias winding for 500ma, fast-blo since it's supposedly a 450ma winding total.

Nothing unusual really except that when I was testing it out initially with that test tone, I bypassed the eq and cranked it balls out for an absolute peak of 32vac into 8.4ohms! The bias probe showed 103ma per tube, I immediately backed it off to idle and it was fine. I figured it was a mistake in the reading since my sockets on the bias tool aren't great.. . Last night at rehearsal I dimed the thing and got glorious chunky distortion for 10 minutes or so and then my center tap fuse kicked the bucket.... obviously we must have hit the ceiling of the fuse and probably maxed the winding out too....

So I'm curious what might be happening here. Is it that my bias supply isn't pushing enough current to keep the UL/DL output setup in check? Bias cap too big to keep up maybe? I'm at a loss because my idle current is fine for my plate voltages, and I'm not getting runaway or red plating. I realize that fusing the CT may be asking for this issue with the fuse, but before I just put a bigger fuse on it, I'm hoping somebody can chime in with advice...

Thanks!!

Jay
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Re: home brew 60watt 2X6L6. ht fuse and bias question.

Postby Anticpunk on Sat Mar 12, 2016 7:02 am

To clarify, the preamp, eq, reverb, mix, PI and bias supply are pretty much dead on to the schematic except that I didn't add the extra 12ax7 for the normal channel, nor did I build in the tremolo circuit.
I have a ppimv in it, switchable negative feedback and a presence control. The only component changes are a 100uf bias cap, a 1k cathode resistor on the reverb driver in place of the 820 that belongs there, 100uf 450v first stage filters and a 2watt 1k resistor in place of the choke. Screen resistors are 470 ohm, 5watters. There is a switch to bypass the tone stack also....
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Re: home brew 60watt 2X6L6. ht fuse and bias question.

Postby pdf64 on Sat Mar 12, 2016 10:42 am

You've been immersed in the amp and know it inside out, but as it's not closely similar to any other amp, it would be very beneficial to others for you to sketch up a schematic. Personally I lose track of text descriptions of schematic x and mods a, b, c ...
Plus you mention that the reverb driver cathode resistor should be 820 ohms but you used 1k; however, the TR AB763 V3 cathode resistor is 2k2, so as an example that illustrates the above point, that there are just too many divergences from a TR AB763 for that reference to be any more than very general.

It's not obvious from the text whether you have used the OT primary screen grid taps for DL operation, or you've left the taps isolated and fed the screen grids via the 1k HT dropper (choke substitute)?

It's a great idea to add a HT fuse, so good call.

Your bias probe looks to be sampling the power tube cathode current. Bias usually describes the dc operating conditions at idle (=no signal = static), specifically the negative dc voltage applied to the control grid in order to obtain a suitable point on the tube's transfer curve (eg plate current/voltage) for the class of operation (eg center biased class A, class AB).
When medium and higher signal levels are applied to the power tube control grid in class AB, its cathode current will increase, as it's now drawing the idle current and the signal current (which is being transferred to the load via the OT).

I hope you can see that is what your bias probe is showing you?

Regarding the HT fuse blowing, I suggest that you put a current meter across the fuse holder and note the current at idle, max sine=clean output, and max square wave=cranked output. It's a good idea to note the HT voltage across the reservoir cap under those conditions too.

Please advise whether you've got a scope and sig gen.

Regarding the ac voltages given, what is the nature of the meter used, eg true rms, averaging?

From the high power output, I suspect that you are pushing the output tubes very hard when the amp is cranked
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Re: home brew 60watt 2X6L6. ht fuse and bias question.

Postby Anticpunk on Sat Mar 12, 2016 3:25 pm

Thanks for the reply! I'll work on a schematic today so I can show the main ideas with the circuit.

I do have a scope and a signal generator.

You are correct about the bias probe. I'm aware of the negative bias voltage and it's purpose (I have a range of -35 to -60 available at the pot), the probe is simply measuring cathode current there. What surprised me was the swing in that current. I'm used to seeing maybe an 100% increase in an amp at max output, but a jump from 36ma at idle to 100+ma seemed very extreme. I am using the DL taps for screens with 470 ohm resistor inline and I'm not so experienced with DL operation.. The tubes do run hard at that max output, but as I said, it's not red plating or anything of the sort, and that's what I expected to see when the draw went up that high....

The 1K in place of the choke was to pull down the b+ string to the next node since I'm not using the choke or a traditional screen supply. I do have a choke for it, but my space is limited.

2 meters, both measure rms voltages.

I will measure total current across the HT fuse holder and report that back as well.

Thanks!!
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Re: home brew 60watt 2X6L6. ht fuse and bias question.

Postby pdf64 on Sat Mar 12, 2016 11:43 pm

As the PT is intended for 4x6L6, the amp will have a stiff HT when loaded with only 2x6L6; I think that the dc cathode current per 6L6 may get around 200mA dc when cranked, ie max square wave; so a 500mA HT fuse may get pushed.
Higher value screen grid resistors, eg 1k5, 2k2, might help to reduce the cathode current which will reduce demand on the PT HT winding.

Bear in mind that the ~500mA rating of the HT winding will be in regard of continuous demand, eg drawing >600mA from it for >20 minutes, alongside the other windings being run at full load, may result in failure. But music isn't like that, even doom metal let's up occasionally and the current drops back, thereby allowing the time averaged load to be within the rating.
That's why the PT is able to supply a TR with 4x6L6.

Fuses for transformer windings are usually rated somewhat above the rated current in order to cope with such peaks, so it seems reasonable for you to try the next size up, eg 630mA.
100 watt Marshalls tend to use 1A HT fuses.
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Re: home brew 60watt 2X6L6. ht fuse and bias question.

Postby ThisLifeILead on Wed Mar 23, 2016 8:49 pm

Anticpunk wrote:I have a ppimv in it.


I installed a PPIMV in a Garnet Deputy plexi copy and i had the same problem when cranked. worked fine untill i had it cranked up. Turns out i had the PPIMV installed wrong and when the master was cranked i lost all negative bias grid voltage. Double check ur negative grid voltage with the amp cranked into a dummy. i forget exactly what i wired wrong but it was in regards to the pot ground wire and the bias feed resistors.
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