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Old 23rd July 2007, 01:37 PM   #1
engels is offline engels  Israel
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Question metal tube lineup for a guitar amp

I'm considering building a 20-something class A guitar amp using some stuff I've got. The PT is from a radio with approximately 182mA of 250V (not CT) and around 4A of 6.3V.

The power tubes are two 6BG6 (=6L6) so I hope to get low distortion under around 280V.

Now I'm thinking about the preamp: I've got one 6SC7 for PI and since I'm sure every design starts from the looks I wanted to use only metal envelope tubes for the preamp. I've got a couple of 6SJ7's and two 6J5's (1/2 of 6SN7). I'm afraid to use two 6J5's because they eat too much current, only one 6J5 maybe. I also wanted a TMB fender tonestack.

The requirements are low distortion and maximum output (I know this does not go together but this is how the amplifiers should be designed, right?)

So what's better:

input -> 6SJ7 ->volume -> 6J5 -> TMB -> PI
or
input -> 6J5 -> TMB+volume -> 6SJ7 -> PI

I believe the first one might work better - I've tried 6SJ7 at the input and it's good but it won't drive a TMB. And 6SJ7 in a second position might cause distortion.

I will probably breadboard the preamp but wanted to hear some opinions before.
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Old 23rd July 2007, 03:08 PM   #2
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Default Re: metal tube lineup for a guitar amp

Quote:
Originally posted by engels
So what's better:

input -> 6SJ7 ->volume -> 6J5 -> TMB -> PI
or
input -> 6J5 -> TMB+volume -> 6SJ7 -> PI

I believe the first one might work better - I've tried 6SJ7 at the input and it's good but it won't drive a TMB. And 6SJ7 in a second position might cause distortion.

I will probably breadboard the preamp but wanted to hear some opinions before.
Your first proposal. Put the 6SJ7 "up front" where you need the gain. Also, the 6SJ7 operates at some very low plate currents, and so doesn't have enough current sourcing to drive the grids of the finals the way a 6J5 can more easily do.

There's a simple Fender axe amp design that uses a 6SJ7 "up front", and it works just great (well, according to the reviews, that is).
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Old 24th July 2007, 08:18 AM   #3
engels is offline engels  Israel
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Yes, I've built a couple of champs and they're awesome! The problem with them is they're not loud enough, the output is somewhere around 3W, not 5.

Anyway, here's what I'm thinking to make. After some digging in the garbage I've found a metal 6C5 which is lowerer current and I'm a bit afraid to overload the PT, so here's the current version:

Click the image to open in full size.
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Old 24th July 2007, 01:00 PM   #4
engels is offline engels  Israel
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Default biasing

I forgot the bias pot!!!!
Probably somewhere on the junction of 6.8K and 250K in the paraphase PI (I've got a couple of 100 ohm 2W pots).
But if there's a better biasing scheme idea I'd like to know about it.


(I've got only a quad of RCA 6BG6GA and they're definitely unmatched)

talking to myself....
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Old 24th July 2007, 04:33 PM   #5
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I think there is a problem with the design of your phase splitter circuit, I don't think you can place the master volume in that location in a paraphase because as you vary the position of that pot the stage will be become very unbalanced - probably to the extent that the you won't be able to control the volume effectively except near maximum volume.. I would place the control after the tone stack and before the 6SC7. 1M load on the stack ought to be ok. You could use a standard style phase splitter such as the ones Fender used in some of their amplifiers. (Your master volume control would actually work with the fender style phase inverter, but distortion performance would be very poor at low volume settings - imo a bit too clever I think.)

Also I am confused by the boost switch.. Something not quite right there either, switch open and you loose the stack, gain goes up of course.. Take a look at Duncan amps site and download the tone stack design software.

Try modeling some of this stuff in LTspice, similar tube types are readily available on the net and should get you close to something that works ok.
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Old 24th July 2007, 06:38 PM   #6
engels is offline engels  Israel
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Default uh-huh

thanks, kevinkr!
I really missed the master volume point! Indeed, it would mess all the balance of the paraphase when it's lower than 250K. I think it shold be removed completely from the circuit - or moved right before the PI, as you suggest.

I wouldn't go for the long tail PI because it's been made too many times and I want something else; 6SC7 is THE paraphase tube after all!

Regarding the tone stack - I just made the same TMB with the switch on another guitar amp and it works awesome, actually I was modelling it on Duncan's tone stack calculator. The only concern is that it was designed to improve sound of one particular amplifier which had too many mids and I was modelling the scoop to cure this problem, which is what it does. Now that amp was 6C4 ->TMB -> 1/2 of 12AX7 -> 1/2 of 12AX7 concertina PI -> 6V6 PP. Too many mids are usually the problem with triodes, and pentodes sound more "scooped" so I wonder if this circuit should have the same TMB.
The boost switch does exactly what you say - removes the TMB and boosts gain for about +20dB, so if the EQ-less sound will be problematical I will change values of the TMB.
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Old 25th July 2007, 08:17 AM   #7
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Default update

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Old 25th July 2007, 04:03 PM   #8
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Looks good, one thing I would change is the configuration of the output jack - probably not a good idea to short the secondary when a speaker is not connected - it would be better to insert a small power resistor of say 50 - 100 ohms. (This should be enough to prevent insulation killing voltage excursions on the primary of the opt if you forget to connect a speaker - with a short very large ac signal currents can flow in the primary and the output tubes and transformer may or may not be damaged.)
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Old 26th July 2007, 01:44 AM   #9
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The shorting speaker jack was somewhat standard practice a few dozen years ago. An open speaker on a P-P tube amp is usually instant death for tubes and transformers. A short will cause a red glow if you play for a while at full tilt, but this usually doesn't happen since there is no sound. It still doesn't help the player who unplugs the cable at the speaker or blows the speaker at full crank. I once witnessed an Ampeg SVT (300 W bass amp) burst into flames when the speakers all failed open because they couldn't take the 300 watts. Most of the 6550's, the OPT and a bunch of small parts were all toast.
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Old 26th July 2007, 06:07 AM   #10
engels is offline engels  Israel
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Default speaker jack

so, what's the adviced solution with the speaker out jack?
shortening or some small value 20W resistor???
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