More 6BM8 madness

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More 6BM8 madness

Postby dhuebert » Wed Dec 30, 2015 6:42 pm

I decided to see what I could do with a couple of 6BM8 in push pull and designed and built this little gem:

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Somewhat busy under the hood:


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This was quite the learning experience for me, still, after all these years I find myself in trouble not understanding what I've done. When I first fired it up there was heavy distortion in every stage. Slowly I sorted out what was going on and ended up reducing the gains significantly. In this form it is still way too hot for what I want, which was a clean, clean guitar amp. What had me going was the bias voltage for the pentodes, I was used to working with 6L6 and KT88, which have a much higher bias voltage. 6BM8 come in at -11 volts which means that the phase invertor can only swing 22 volts max peak to peak. I had built a single ended 6BM8 amp that made 3 watts so I expected this one would make around 12 watts. It don't. 4 watts is as much as I can get out of it. The output transformer has lots of taps to customize impedance so I tried them all and maxed out at 4 watts into a 10 ohm resistor. The challenge now is to figure out why it's not making more power.

Don
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Re: More 6BM8 madness

Postby EWBrown » Wed Dec 30, 2015 9:40 pm

Two questions - What is the screen grid voltage for the PP power section? It could be too low
(I don't immediately remember that particular Zener diode's operating voltage).

OK, I looked it up, 110V, 5W. Might be to low - I usually run them around 200-220VDC, or even Ultralinear.

Also in the cathode voltage for the PI stage, the LM334 might not play so well with the 6BM8's relatively high-mu triodes,
so the shared cathode current may not achieve the desired 3 to 3.5 mA. The LM334 may have to be referenced to a negative voltage,
even -5VDC should suffice. I had a similar situation with one of my early 6EM7 PP amps, my PI circuit topology was nearly identical to yours,
and the LM334 was a bit "squirrely" due to insufficient voltage, so I changed it from zero volt (signal ground) reference to -6VDC, that really helped it
increase the power output.

A PP pair of 6BM8s with 250VDC on the plates should normally deliver 8 to10 W RMS. With an 8K 0r 10K A-A OPT.

/ed B
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Re: More 6BM8 madness

Postby dhuebert » Thu Dec 31, 2015 8:32 am

Thanks Ed, I knew I could count on you.

I took the zener voltage for the screen grids from a circuit I found somewhere while teaching myself about mosfets. I'm going to try taking the zener out and see what effect it has on power. Years ago I saw an article where a guy had gotten 60 or 70 watts out of a pair of 6BQ5 by making the screen voltage low and the plate voltage high so I kinda wanted to experiment with that.


The PI works perfectly. I investigated it pretty closely at first because it was clipping pretty hard. Turns out it was the -11 volts on the grid of the pentodes that was taking the tops off the PI output. I will look at the voltage on the LM334. You have me curious.


A PP pair of 6BM8s with 250VDC on the plates should normally deliver 8 to10 W RMS. With an 8K 0r 10K A-A OPT.


That's what I was thinking. This circuit has over 300 volts on the plates so maybe 12 watts? I tried all the combinations on the output transformer and got the highest power with 11k.

The worst thing about it is that the front end buzzes. I used all the same techniques I've used on my other amps to get them quiet and it still buzzes.

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Re: More 6BM8 madness

Postby EWBrown » Thu Dec 31, 2015 6:02 pm

The buzz indicates that you are picking up a stray EM /RF field, most likely culprit is a switching mode power supply or light dimmer on the same
AC power line circuit branch. Check the input jack shielding and grounding, that's the most likely point of electrical noise ingress. That input stage has a fair amount of gain, and a relatively high input impedance (as usual for guitar amp designs). Hum and ripple are Power Supply deficiencies or ground loop issues, buzz is generally an external "invader"

I've always used 250vdc (+/-10V) for my 6BM8 / ECL82 / 6F3P / 11BM8 / 16A6 amp designs, in order play it "safe", but then I run them about 35 mA cathode current on the pentode section. With 300VDC, drop the current down to 27-30 mA so that the plate dissipation isn't too grossly exceeded, and the tubes should remain "healthy" for normal lifetime of service.

I've always used cathode bias in my designs, rather than fixed bias, so YMMV under full power operation.

Look through my past postings about K-502 circuit mods and hacks, and 6BM8 SE circuits.

As I recall, the Vox AC30 (or was it AC45) also used PP 6BM8s in a relatively basic small 10 watt amp design.

/ed B
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Re: More 6BM8 madness

Postby Gingertube » Mon Jan 04, 2016 9:20 pm

G'day Don,
As ED says the low screen voltage is probably reducing the power significantly. The Pentodes Bias volts required will increase when you increase the screen voltage.

I'm also worried about the 20K Ra (the anode load rsistor) on the 12AT7 stages. That will cause a lot of distortion.
For guidance on this: As the current in the tube changes then so does the internal rp. The change in rp value with signal current swing compared to load resitance IS the distortion level, so keep Ra value up for lower distortion.

A handy reference for recommended Resistor Values and what Gain you can expect in Resistance Coupled Amp Stages:
http://www.tubebooks.org/tubedata/hb-3/ ... ifiers.PDF

The 12AT7 data is about the 6th page. You will note it only covers Ra values of 100K (0.1M) and up. It assumes that no-one will want to use values lower than 100K

If I'm being OCD today then I would also say that EVERY triode should always have a grid stop resistor even though a lot of amps schematics don't show them, in fact there a re quite a few amps which do have them (gridstop resistors) but the schematic does'nt show them.
The minimum recommended value of a gridstop resistor from the long since safely deceased guru's of tube design is equal to 8 divided by the triodes gm. gm for 12AT7 varies with operating point but from the datasheet it will typically be greater than 4mA/V.
Calc: 8/0.004 = 2000.
So minimum of say 2K2 on 12AT7 (compared to minimum of 5K1 for 12AX7).

gm for 6BM8 triodes is approx 2.5mA/V so minimum 3k3 grid stops for those.

When you ditch the zener on the screen supply you will probably want to drop the value that 15K too. 15K will give you a lot of compression altghough you may like that.

EDIT: Also - next time you buy shielded tube sockets DON'T buy those shiny silver things. They concentrate the tube heat back into the tube. You want the ones which have the black inside and outside of the shield to first absorb the tube heat and then radiate it from the outside. The shiny silver on the shield inside reflects the tube heat instead of absorbing it and the silver outside is crap at radiating what little heat is absorbed. Using the shields with black inside and outside make a surprising difference to tube temperature. Your tubes will last longer. Check out some of the old Tektronix or Hewlett Packard Tube based Test equipment - full of shielded sockets on which the shields are black inside and outside.

Cheers,
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