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Old 2nd March 2004, 02:56 AM   #1
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Unhappy Phase splitter questions

Hi, New here and new to tubes. I have a degree in electronics, They didn't teach tubes anymore when I went Here is my problem. My phase splitter seems to be gating(or something). It will not produce output with less than a 18V P-P input. Sounds great with the preamp cranked full power. The problem is trying to play clean or even softly(By the way home made guitar amp). I will try to post a sad looking schematic of it. If someone could take a look at it and tell me what you see I would be very greatful. Any comments are welcome on any part of the design.
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Old 2nd March 2004, 03:43 AM   #2
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Ok, see if this works. Schematic in Zip format.
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Old 2nd March 2004, 03:25 PM   #3
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You've got 212 volts on the top plate of the phase splitter and 135 volts on the bottom plate. Something is wrong here. These should be within 10%. My gut feeling is you're not pulling enough current through the phase splitter. I would go with a lower value tail resistor (the 47k).
See the Fender 5F6A bassman for a nice sounding phase splitter for 6L6.
http://www1.korksoft.com/~schem/fend...5f6a_schem.pdf
The Fender AB763 phase splitter is good too.
http://www1.korksoft.com/~schem/fend...b763_schem.pdf

6L6 screen resistor:
100k seems kind of high here. Most common is a separate 470 or 1k resistor to each screen. I use 1k / 5watt resistors here.

2nd and 3rd gain stage:
I noticed the 250k plate resistors. Do you really need all that gain? I would reduce the plate resistors to 100k. Use separate 1.5k cathode resistors on the 2nd and 3rd stage. You might want to leave the 3rd stage without a cathode bypass cap. Keep the 470k grid stopper resistors, that's a good idea to prevent blocking distortion.

I rarely use more than a 4.7uf cathode bypass cap in guitar amps. Often, I use 1.5k / 1uf or 2.7k / .68uf. This will cut bass response and make a tighter sounding amp.

I would move the master volume control to after the 3rd stage, before the phase splitter. this would give you the following topology:

gain stage -> gain control-> gain stage -> tone control -> gain stage -> master volume -> phase splitter -> power tubes.

You should be able to get a good clean sound out of this as well as a high gain sound, and everything in between.

Tone control:
You have a simple high cut tone control here. You already have a high cut control on your guitar. I prefer the more usual bass, treble, mid controls, myself. You may be limited by available panel space. Here is a site with a good variety of tone controls
http://amps.zugster.net/articles/tonestacks/

Gain / master volume pots: these are 500k pots with 470k in parallel. I'm not sure what you're trying to accomplish here. Most guitar amps use 1Meg audio taper pots without parallel resistors.

Hope this helps,
Mike D.
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Old 2nd March 2004, 05:31 PM   #4
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Yes, you want to run around a milliamp per section for that phase splitter. The plate voltages should end up about 15-20V apart. For a guitar amp, I'd probably not worry too much about AC balance and use the same value resistor on each plate. Also, you don't indicate any grid leak resistance between the o/p tube grids and the bias pot. Hopefully, that was just something omitted from the drawing.

What you're doing with the feedback is confusing me- why are you running it back to a common-mode point?
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Old 2nd March 2004, 10:04 PM   #5
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Default Thank you

This is my first amp(besides modifications of others). I will try the things you suggest. yes the tone control is due to the amount of pots, I started with a McMartin PA amp. That tone circuit is just a slap job for testing, I will improve it when a get a little further. The 470Ks across the pots are to keep it from motor boating.

Yes the gid leak resistors are omitted
The feed back is modeled after Marshall/Matchless.

What should the output transformer ohm out to for these tubes(primary and secondary)?

Thank you again for your help, Nobody I know knows tubes so it means alot to me.

Tim
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Old 3rd March 2004, 03:49 AM   #6
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You'd do well with roughly 6600 ohms plate-to-plate for the o/p transformer. The secondaries typically have multiple taps for different impedances, and here it's a matter of matching to the speakers you're using.
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Old 3rd March 2004, 07:51 AM   #7
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OK, rewired and can now play clean and at low levels(Thank you). Cranked up it sounds real bad(as in not good kind of bad). This is why I ask about the output transformer(As the waves going into the power tubes look good on the scope). It measures nowhere near that high(around 450 Ohms). As I said it was an old PA. It said it was 35 Watts. The way it is wired now should be around 50 Watts,Yes? I started to think about this and I think I am getting primary coil saturation or have a bad transformer. Does this sound right to you? Is a new transformer in order?
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Old 3rd March 2004, 09:35 AM   #8
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Hi,

Quote:
It measures nowhere near that high(around 450 Ohms).
Could it be you just measured the DC resistance of the primary?

Cheers,
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Old 3rd March 2004, 09:18 PM   #9
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Yes No way to measure with ac on it.
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Old 3rd March 2004, 11:27 PM   #10
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Tim,
I also have a McMartin PA, model MA-50. I bought it at a hamfest for $25. I converted it into a bass amp. The one I have runs the power tubes at a savage voltage of about 625v. It used an OD3 gas tube to supply a screen voltage of about 400v. I ditched the noisy OD3 and made a semi-regulated screen supply with a couple of TIP50 transistors. I run el-cheapo Sovtek 5881s in there with no problems in 4 years of weekly use.

Anyway getting back to your transformer, you can do a poor man's impedence measurement with a variac and a voltmeter.
see http://www.gbronline.com/radioguy/outimp.htm

Make sure you are using the correct secondary tap. PA amps have a 70v secondary tap for distributed speaker systems and this tap does not sound too stellar for guitar speakers.

Assuming the transfomer is ok and it still sounds bad cranked, it could be crossover distortion. Your schematic shows 450v on the 6L6 plates. Set the bias so that each tube is pulling about 30-35ma. This biases your tubes at about 50% of their max plate dissipation of 30 watts. That's a good starting place for bias in a guitar amp. The PA amp is normally biased much closer to class B and does sound bad cranked.

OK, we're using the correct secondary tap, bias is set, does it still sound bad? try disconnecting the NFB. Still sound bad? maybe some oscillations. Try a 47pf cap accross the phase splitter plates like the 5F6A Bassman.

Still bad? Well it could be a bad transformer. I've seen it happen. I fixed a gibson stereo guitar amp where one channel sounded like poop and the other was marvelous. I really beat my head against the wall until I swapped the transformers and discovered the problem was indeed the transformer.

Regards,
Mike D.
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