Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Prento on March 12, 2025, 09:15:11 pm
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I am building an approximation of Sluckey’s Tweed Deluxe Reverb. Basically it is a three triode preamp with the master volume feeding signal to the PI, so the Tweed Deluxe reverb without the reverb.
I have a few questions.
Firstly, between the second and third triodes is where the reverb circuit is inserted via a 3.3m resistor paralleled with a 10pf cap. As I am not having reverb, should I just connect the 22nf coupling cap directly to the third triode, with perhaps a grid resistor to tame any oscillation or should I include some kind of a voltage divider to reduce the strength of the signal hitting that third triodes? (I don’t have an oscilloscope).
Secondly, the master volume on the schematic is 100k. What is the reason for such a small pot?
Thirdly, am I right in assuming that the 220k resistor works in conjunction with the 100k pot to reduce signal to the PI.
I know they may seem like silly questions but I’m just not sure.
Thanks
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Here's the schematic and layout.
Tweed Deluxe Reverb Sluckey (https://www.sluckeyamps.com/tdr/tdr.pdf)
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Not silly at all. The question about how to delete the reverb circuit has been a tough one for my builds.
First, recognize that three triodes is a lot of gain. You will have to tame it to your taste. I would keep R21 as a grid leak resistor, but might change its value (probably smaller). (A scope is really helpful for this, and a small handheld scope will work for less than $40 on Amozonia. )
This is a useful reference: https://valvewizard.co.uk/Common_Gain_Stage.pdf (https://valvewizard.co.uk/Common_Gain_Stage.pdf). Personally, I've had problems with grid-current clipping (see p 19) in just this situation, and this is adjusted by grid and grid-leak resistors.
Some the more erudite members may be able to explain better.
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The simplest solution may be to delete the 3rd triode and build it like the normal channel of an AB763.
There's other options to use 3 or 4 triodes to try and get more gain.
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Prento what kind of sound are you looking for? Clean blackface? Ability to get some distortion?
Maybe consider a JCM800 preamp married to the AB763 power amp. Look at Robinette's creations.
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You might get some ideas from Hoffman's Blues Jr Conversion project. The voltage divider values should get you in the right ballpark.
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Thank you for your assistance gentlemen. Reading your comments reminded me that many of the more modern fender reissue’s schematics include AC voltages for grids and plates. After looking at these I’ve decided that through trial and error I can create a voltage divider between the second and third triodes to get me where I want to be and avoid any nasty effects of too much signal passing to the next stage and onto the PI.
To address some of your questions, I’m looking for that black panel sound with the ability to up the gain if needed. I have put in a switchable nfb and a 25k mid pot in series with the 6.8k mid resistor. I don’t have reverb but I have grafted in a bias vary tremolo circuit from the 6g3 Deluxe. This amp is actually for a mate and he also wants to be able to play quietly. It’s all a bit of an experiment. The chassis is a 1980s Peavey Bandit that wasn’t worth fixing. As for the power supply, I’m using a transformer the origin of which I’m uncertain. It puts out 135vac and I have coupled it to a voltage doubler, I’m using a separate transformer for heaters. Under load it gives me 350vdc and doesn’t get overly hot. Here in Australia designated guitar amp power transformer are much more expensive than in the USA and sometimes you need to be a bit imaginative. The output transformer is a line output transformer manipulated to work as a standard output transformer, a quite inexpensive alternative to the real thing. As you are all well aware transformers, then valves (tubes) are the most expensive components and once you have them sorted, caps and resistors are relatively cheap.
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I'm thinking of building a Princeton Reverb with Reverb and Tremolo deleted, following Rob Robinette's layout: https://robrobinette.com/AA1164_Princeton_Reverb_With_Reverb_and_Tremolo_Deleted.htm
That shows the components left in to compensate for the removal of the reverb circuit - it might be similar to what you are looking to do, blackface Fender sound with 3 preamp triodes.
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Thanks Dogburn. It’s interesting that he maintains the values in that voltage divider and dumps so much signal to ground. It never occurred to me to maintain those same values as I thought dumping so much signal was going backwards even though I knew I had to dump some. That gives me a starting point at least.
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I thought I’d start with a 50% signal dump. I think I may have to increase that to 60% dump as on maximum settings there is a bit of instability which I think is due to too much hitting the PI. I have also removed the cathode bypass caps from the second and third triodes to try and reduce noise and gain. At lower settings it sounds great.
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I had a chance to heat the soldering iron today. I changed the grid resistor in the voltage divider from 100k to 150k, the resistor going to ground is also 100k, so I went from a 50% dump to 60% dump of signal into the third triode. This didn’t make too much difference with everything dimed so next time I will try a 220k which will cut the input to 31.5% of the original signal, a dump of 68.5% of signal. Dogburn’s princeton example from Rob Robinette, a couple of posts ago, shows only 6% of the original signal going to the third triode. I hope I don’t have to go that low.
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I’ve settled on a 470k grid resistor with 100k going to ground. This voltage divider sends 82.5% of the signal from the second triode into the third to ground. I was actually quite amazed that I had to dump so much before the amp would behave, but it has allowed me to put the cathode bypass cap back onto the first triode and I may yet also replace the cap over the second triode’s cathode.
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The simplest solution may be to delete the 3rd triode and build it like the normal channel of an AB763.
There's other options to use 3 or 4 triodes to try and get more gain.
Well I’ve taken this advice as I was not completely happy with the sound having the third triode. It sounds spectacular as Leo designed it. But, there’s no such thing as a free education and the fun is in the learning.
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Compare the topologies of the trainwreck amp. You will see what your describing closely resembles that one. They have already taken the guesswork out of it.