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Author Topic: Started out as a P-P Champ, sort of  (Read 2367 times)

printer2

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Started out as a P-P Champ, sort of
« on: April 17, 2016, 08:35:14 PM »

Had a 12AQ5 P-P amp with mosfet PI that I put in a small chassis and was planning on putting it in a narrow panel Tweed Champ cabinet. Had it with volume and tone control controls along with a switch that I used to change the NFB. Eventually I tried adding a bass control that could be switched in that was to replace the NFB switch.



Found it did not do too much, mind you both the tone and bass control should have entered the top of the volume control, I had the tone (treble) still hooked up as the treble bypass Tweed style. So it got me thinking, convoluted path to the following schematic. The first stage using the LND150 or 2N5485 with the first stage of a BSIAB. I have not decided which way to go, going to have to breadboard both as the two drawings I pulled together from posts on the web.



Have some numbers that are a little useful (to me at least). Looked up some Fender schematics and came up with the gain of a 57 Bassman RI sections using the Amp's service manual voltages.  Input is voltage gain of 50, second stage 41, the tone stack -0.6. The PI has a gain of >1 and the 12AQ5's need 15V for full output. First stage should have more than enough gain with the second stage biased at 1.25V. The second stage gain of 41x being hit with a 1V signal should put out 40V. Cut that in half through the tone stack and you have 20V to the 12AQ5's.

So depending on tone stack positions the second stage should start to clip around roughly when the outputs do. Have the amp running on 240V right now (Tweed tone controls separating two triodes) but am thinking of going with a voltage doubler for roughly 300V with the transformer being rewired. Have a 70V transformer for the output, if I am lucky I would guess 7W out of it. Just want a small practice amp out of this one and using it to try out something different.

So that is the theory by me, may not have anything to do with reality. Just wondering if anyone sees something wrong with the concept.
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J M Fahey

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Re: Started out as a P-P Champ, sort of
« Reply #1 on: April 17, 2016, 10:42:33 PM »

Cool concept  :dbtu:

Just a couple questions:
1) I guess you will have way too much gain at the first stage with the LND150 , you don't want more than ~50X tghere and I bet yiu are way beyond that.
Of course, measuring is King, so actually build the stage and check  :)

2) don't understand the 1k/9k1+diode net between 2nd and 3rd gain stage, impedances surrounding them are way higher and will dominate the design.
What are you trying to achieve?
FWIW that net is fed through ~40k impedance; and the 9k1 is away from the circuit ... until grid becomes 0.7V more positive than cathode.
Please label parts, to avoid finding them by "walk 2 blocks, turn left on the traffic light, pass the 3rd tree, it's the scond green door" type instructions, it's easier to say "R1 ... V3 ...." etc.  ;)

3) as of cascode Fet stages, I HATE them with a passion, they have nothing to do in a MI amplifier, they are designed for instrumentation, oscilloscopes, etc. ; Jack Orman should be stoned for making them popular, no kidding (and claiming as his own a stolen design).
Rant over.

Please post results.
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printer2

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Re: Started out as a P-P Champ, sort of
« Reply #2 on: April 18, 2016, 07:51:56 PM »

Thanks. Thought of where I could get the most out of the two triodes, first stage not so much. In front of the PI, maybe but I might have more gain than I really want. What the heck, try it in the cathode follower and see what I get. What I put up front is still up for grabs. The mu amp, I remember seeing it in National Instument's books in the 70's and actually thought of trying it for guitar but I guess I was too busy and never got around to messing with fets.

Still have not and the only experience I have is through Youtube and I didn't mind some versions too much. I thought it might be interesting with a low gain fet but I think the lowest I have is the 2N5485. If one fet has enough gain to get where I want to go I'll just as soon do that, I am tight enough for space in my chassis. I would rather save my limited number of LND150's for other duties. I'll probably try one to see what they can do though.

The 1k/9k diode is something I picked up at Merlin's site.

http://www.valvewizard.co.uk/dccf.html

No idea how much stress the tube goes through without it but I put it on the last amp I built with a CF. Makes for an interesting arrangement on the socket and if it is unnecessary I can live without it. R1, C3...  ...that takes the romance out of looking at schematics. :)  I'll see what I can do. Not many people comment much when I post my ideas other than, you know I don't get too many comments. Anyway I will for sure post how things go. Now that I have broken down the barrier of using sand in the signal path (other than as a source follower) there really is no stopping now. Might even do a whole non-tube amp.
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printer2

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Re: Started out as a P-P Champ, sort of
« Reply #3 on: April 22, 2016, 09:19:17 PM »

Thought I would have the amp rewired by the end of today. Rather than that dipped my toe into the LTSpice world and came up with the following for the first stage of the amp. For some reason I thought I could get more gain out of 2N5485, best I could do was 15X. Would have liked to do it with one stage, space is a little tight.

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printer2

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Re: Started out as a P-P Champ, sort of
« Reply #4 on: July 22, 2016, 12:11:42 PM »

Still on my bench waiting to get finished, still whittling on wood instead. Saw someone on diyaudio.com tried to get the Tweed bass and treble controls to work better, saw the beginnings of a James tone control. Probably would be better with a three pole switch but the whole point was to use a bass control with a push-pull switch on it.

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printer2

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Re: Started out as a P-P Champ, sort of
« Reply #5 on: July 24, 2016, 01:40:47 PM »

What the heck, nothing better to do with my life. If you look real close you can see a 5E3 Deluxe happening, well that is if you look real close and turn your head sideways.

 

Love these mini four pole toggle switches. In the as shown position the circuit is an octal 5E3. The way I figure it most go through the bright channel anyway and load the channel with the normal channel pot. Really does not matter if the 'Normal' channel has a Pentode or volume pot as a resistive divider. You may loose out loading the bright channel from 500k to 1M, not a great loss. Otherwise you can turn the pot down to zero loading down the other channel. If you want to use the 6SJ7 channel at some time all the power to you.

Now say you want to try something a little different from the 5E3 dirt, how about a James control tone stack going into a pentode? Should give a few more tone options. Have it going into a single ended output, I have a lot of SE OT's. Maybe one day...
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printer2

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Re: Started out as a P-P Champ, sort of
« Reply #6 on: August 12, 2016, 12:24:24 PM »

One more (all ideas for winter projects). Thinking of having this in a large pedal enclosure and running a switching amp after it. Have a switching module for the HV.First thing, forgot to put diodes to protect the opamp input. S1 is a clean/dirty switch if used that way or to run as Marshall's higher gain series amps. With S2 engaged a Plexi flavor. 



So what is wrong with this picture?

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printer2

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Re: Started out as a P-P Champ, sort of
« Reply #7 on: July 02, 2017, 01:27:36 PM »

Yeah it has been a while, was not 100% sure of the direction. Phase inverter back was fine, decided to just do two triodes as gain stages and a volume and tone between as a Tweed Princeton. I have done a circuit with a bass and treble stack that switched to Tweed tone control for the treble control in another amp that worked out well. Thought of doing that but then I had the idea to try bass/treble tone control in one position and 5E3 funky controls for the other. With only one input the second channel is sort of simulated by the bass control with a resistor on the wiper to act as the tube plate.

The volume control gets flipped with the wiper on the plate and top lug with the bass control top lug together. That leaves the tone control. The top capacitor in the stack and Tweed control work out to be different according to Tone Stack Calculator. I'll tweak that by ear. The bottom cap is filled by the 0.010uF and 0.033uF caps in series. Should function well enough, I think. Otherwise there is the NFB for BF mode, will adjust resistor to suit also. In theory it might work.



As you can see, photobucket finally decided to squeeze the people they offered free accounts to. Goodby photobucket, hellow imgur.
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printer2

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Re: Started out as a P-P Champ, sort of
« Reply #8 on: July 15, 2017, 09:45:29 AM »

I found the 5E3 thing with the bass control acting as the second volume control did not do anything major to the sound. I decided to go back to an earlier idea about using a 500k pot with a push pull switch to convert to a bass treble tone stack to a sort of Tweed tone control. I dropped it when I realized that the bass control can effect the operation if it is not set for maximum resistance. Since I had the circuit breadboarded I thought might as well see how much effect the bass  control had on it. I found the control increased or decreased the sensitivity of the tone control and did not seriously muck up the operation of the tone control. So I went for it and wired it up.

I also found I had different values in the amp as were on my earlier schematic. I updated the part values, still need to hook up the NFB. Actually a nice little package with the small cab and the 8" speaker. I am going to leave it as is for a while but I would like to see if I can push the voltage up to around 300V. Sorry for the big pictures again, still working things out since I dropped photobucket.







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