Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on October 24, 2016, 09:18:30 pm
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Spent some time tying to decide a direction for a very personal preference type of amp. I have been wanting to do something with both reverb and trem. I ended up playing a Princeton in a shop recently and really loved it. It was just so amazingly clean. However, I like a combination of great cleans with the capabilities of rocking lead tones. After looking at the Princeton schematic, I felt I could have made some easily switchable modifications to achieve the extra stuff I would like to have. I have seen a few designs that add in an extra gain stage by cascading it into the circuit when in use and by just letting it float when not in use. Meaning not grounding out the grid when not in use. The Mesa Lonestar is an example of this. I have devised a way to do this with the Princeton by using either a DPDT switch or relay. If I use a relay I can use an optocoupler to ground out the grid when not in use by connecting the extra gain stage to the normally closed section of the relay. When the switch is flipped and the relay kicks on, it also sends power to the optocoupler opening the circuit and grounding the grid.
I have swapped many Bass/Treble tone stacks out with a marshall TMB stack. I did the same here. My question is, how should I go about optimizing this stack to function at its best with this amplifier design? I know there are tone stack calculators out there. I have never used one. Does it really change all that much or does the cut and paste method work fine?
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I can tell you that adding a gain stage to a Princeton can turn it into a flaming monster. I changed the output section in mine to dual 6L6 and converted the trem tube to just one single triode gain stage; you may or may not wish to do that. At the time I did this, I knew nothing about calculating gains, I just pretty much duplicated the first preamp stage and threw it into the chain after the tone stack. My purpose was to create a "pre" gain and a "post" gain. Sounds kind of like what you are contemplating. Well, you could say the gain went up...I did not get the parts values right but that amp was capable of very, very loud cleans and borderline out of control nasty high gain. I really could not turn it over about 3 without forcing everyone out of the room. It was a great sounding amp. Along with the 6L6 change, I used a Dyna Mk IV transformer which no doubt was capable of driving a speaker with extreme gusto, real 35-watt OT.
It sounds like you have done a fair amount of experimenting, more than I, with the tone stack. But what I would say is that if you want loud-clean, louder than how you play in a store, I'd imagine that with your extra gain stage you would be overdriving 6V6's early and easily. And a PR OT is already undersized, which has a lot to do with how much tone you can pump out which would make your super-tone-stack seem more worthwhile.
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> does the cut and paste method work fine?
When in doubt, plagiarize Leo.
33K is an unlikely low value for a tonestack driven from a plate(s).
Heed Mr Teen's notes on gain. Some gain is good. Some*some can make a monster than can't be played in town. One stage may be gain of 50. Two stages is 50*50, or 50 times more. The difference from clean to rocking isn't as much as 50. "Booster" stages usually need some added loss. Gain-up, distort, drop-down, gain-up, etc.
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I have been wanting to do something with both reverb and trem. I like a combination of great cleans with the capabilities of rocking lead tones
I'll offer this for your consideration. I have not built this amp but it's one I'd love to build. IF I were building it, I'd use a PT with at least 140ma and maybe 300-0-300. With a PT like that one could use 6V6 or 6L6 without rebiasing like I do in both of my current amps (given this is cathode biased).
I think you could get stellar cleans and the boost features could get you into a more overdriven tone fairly easily, IMO. You could use
Doug's relay switching for the midboost and PAB.
If you haven't played an amp with a dumblish type tone stack, the mid boost and PAB (preamp boost) add considerable overdrive.
I do have a layout drawn up for this also.
With respect, Tubenit
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Thanks Tubenit! Thats a very interesting design. Im going to breadboard my original schematic and play around with some modifications. I'll study this layout and maybe try a few things. Can you tell me if adding the gain stage like I did could affect my tremolo or reverb?
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I did not get the parts values right but that amp was capable of very, very loud cleans and borderline out of control nasty high gain.
Did you eventually get the amp tweaked like you were hoping? I want to build the amp based off of how amazing this amps sounds as is. Stock is great. I jammed one in a shop and loved the cleans. After spending some time working on a Mesa Lonestar, I got the idea to try and add a gain stage the same way they did in that amp. I love the extra gain that comes with it.
With your conversion of the reverb, using only one triode rather than one and a half, what did that entail? The first two are just in parallel so I am assuming it was as simple as just cutting out the one half? I don't know much about reverb circuitry though. Also, with your conversion, did you still get the reverb results you were hoping for?
"Booster" stages usually need some added loss. Gain-up, distort, drop-down, gain-up, etc.
I am expecting quite a bit of tweaking and calculations for the extra boost there. The Lonestar for example is driving a tone stack, so theres loss there. My values in the schematic were mainly for schematic purpose (to complete the schematic just to show my direction). I will be picking everyones brains for ideas on values when I get this on the bread board :icon_biggrin:
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A PR begins to get dirty when cranked over 8/10ths on the dial. But it has heaps of untapped gain in the existing dry signal path (which is otherwise thrown away to get that bucketload of BF reverb) that is heavily attenuated by the 3M3/470k voltage divider in front of the wet/dry ('mixing') stage. A simple mod to return this gain to the dry signal (which will remove the almost all the reverb from the mix) without needing to redesign the amp, is to put a shorting switch across the 3M3 resistor.
An elegant solution would be to use a switched pot - i.e. replace the existing 1MA vol pot with one of these:
(http://i.ebayimg.com/images/g/qI4AAOSwB4NWxjq7/s-l300.jpg)
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"Did you eventually get the amp tweaked like you were hoping? I want to build the amp based off of how amazing this amps sounds as is. Stock is great. I jammed one in a shop and loved the cleans. After spending some time working on a Mesa Lonestar, I got the idea to try and add a gain stage the same way they did in that amp. I love the extra gain that comes with it. "
Frankly...no. This was 25 years ago. I did not have a bench at the time; Nor did I have any idea of which direction I should change parts values to get a more controllable situation. I kludged it together on top of a table saw with junk parts...Out of pure laziness I just kind of learned to use it how it turned out and came to respect that my "pre" gain had 3 settings: Off, usable, and ridiculous out of control, all of which occurred between "2" and "3-1/2" on the "pre-gain" knob (!)
Gradually the amp began making so much noise I could not use it. Then the amp went into storage for 20 years and since I had never made any drawing of what I had done, I just ripped all my mods out, recapped and re-plate resistor'ed it and went pretty much stock, except of course with the 6L6 output tubes, the Dyna OT and the bigger PT. Tremolo is gone. It is a great sounding amp. I am very confident that the idea is quite valid, that a "pre-gain" stage with very modest gain (guessing) 2x-5x would make the amp much better without being Godzilla.
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There is a great tutorial on the difference between Fender and Marshall tonestacks. You might find it interesting as well as useful. Here's the link:
https://www.ampbooks.com/mobile/classic-circuits/bassman-tonestack-intro/ (https://www.ampbooks.com/mobile/classic-circuits/bassman-tonestack-intro/)