Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: acheld on February 26, 2025, 04:24:44 pm
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All you need is an excuse, right? So I have some unused iron and old chassis -- all begging to be used. So, I'll oblige!
This amp goes back about 15 years or more, originated with DaGeezer -- and is widely acclaimed over the years. Tubenit has also built this, providing his flavor to the pot.
For me, it's a take on Dumble style overdrive, able to deliver beautiful cleans when asked, and creamy overdrive when needed. Personally, I leave the OD section engaged all of the time.
Attached is my schematic. Apologies that I could'nt fit the circuit on one page . . . All thoughts are considered.
There will be tweaking at a minimum.
Issues for the design: I do not enjoy the Dumble PreAmp Boost. While it works, it is noisy, and I have never been able to tame that aspect of it. Instead, I've used the "Mid-Boost / Deep" -- but here in conjunction with a fat switch (to be rolled into one relay) to accomplish the mission. I'm not sure this will work. I would like a boost to be used soloing. OD is "on" all of the time.
One question I've had -- the Hammond 290CX I have has a 5VAC tap. Could I use this tap with Doug's power supply board to deliver 5 VDC? I'm not sure if the rectified voltage will be enough for the LM705. Not sure why I even care about this, as I have plenty of heater circuit juice to drive an LM705.
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Hi acheld
"heater juice" would be almost sufficient to drive LM7805T (6.3x 1.41=8.82VDC subtract 2x Diode drops say .7V ie you would have approx 8.8V - 1.4V=7.4V DC, good design requires min of 2.5V DC in excess of regulated output. (3V is a better figure so that regulation is guaranteed)
BUT!!! you are switching this via XLR Conn and using an earth!! You just got into trouble (Jail?) :help:
I would use the 5VAC spare winding, fit a voltage doubler and your are looking at min 12V DC (after drops/losses) going into LM7805T and the relay current is not high so no heatsink required and again Do not earth.
I like you HT section :headbang: and don't forget the Heater to ground treatment Earth CT or 2x100R to ground,
My thoughts on PS requirements :icon_biggrin:
R306, R307 min 1Watt MF or Metal Oxide for longevity.
Kind regards
Mirek
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Thank you for the comment. You are correct about the earth symbol being totally wrong. I did not pay attention to my drawing. Next version will have a complete drawing of the relay system, which I have lovingly lifted from the Dumble schematics out there, and used with success.
The XLR connectors are far more robust than the tiny DIN sockets.
In the past, I've always thrown in a tiny 6.3 volt transformer, and used a doubler for an LM 7812, but this time I thought I would try one of Doug's 5 VDC boards for the relays. But I like your idea of using that 5VAC source with a doubler into a 7805.
290CX does have heater center tap, which I will be using.
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I look forward to following the progress on this build and hopefully hear sound clips from you! :icon_biggrin:
I seldom use the PreAmp Boost, but this is what I've settled on for my PAB. It's the best I've come up with.
More moderate boost in volume and perhaps a little less high end harshness leaving it a little smoother in tone, IMO.
Not trying to talk you into experimenting with this, simply making mention of it for anyone who might want to try a PAB in their amp design.
With respect, Tubenit
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I will give that PAB a try. Much appreciated.
In using various Dumble clones, the PAB has usually been noisy -- and annoying -- but it does what it says.
The Mid-Boost/Deep gives nice, but limited, boost. I was trying to think of a way to take it up a notch, when I thought of adding a Fat switch into the mix. It might work, but may add too much low end. It's a kluge, and I was looking for an alternative.
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Updated schematic for TBM 2025 -- this includes mapping for the relays as well as proven working relay activation circuit.
Time to build!
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Back to this build, which is now making some progress . . .
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Build is up and running, but I have a big question regarding voltages. I haven't listened to it yet.
Full schematic attached, as well as single sheet with just the power supply.
Line voltage is 121VAC. Tube complement: V1-12AY7, V2-5751, V3-12AX7, V4/5- 5881
B+1: 453 VDC
B+2: 452.9
B+3: 432
B+4: 309
B+5: 303
I think these plate voltages are just too high. Although I have operated 6L6's around 400+VDC, 450 seems excessive.
And for sure the PI 12AX7 (B+3) is way too high.
For the moment, the 5881's are biased very cold -- around 70%, but when biased at 105% the B+1 was only 15 V lower. My intent is to bias around 90-95%. So that buys me about 10V lower.
I do have an option of ditching the SS rectifier and moving to a GZ34. It will mean more drilling, but I'm ok with that.
Opinions? There should be no lack of those . . . :icon_biggrin:
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How about ignoring the high plate voltage and dropping the screen voltage as seen in the Fender Super 5D4?
Leo used a 10k power supply dropping resistor instead of a choke between B+1 and B+2. The layout view shows it dropped 90 volts!
I have not tried that approach but do use a 5k/5watt on my tweed deluxe in that position with 6V6GT power tubes.
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My understanding is one can use a resistor in series with a choke to lower voltages? Hopefully, someone more knowledgeable can answer that question.
The voltages on the 5881's would not concern me. You could use a 12AT7 in the LTPI if you wanted to? The tube rectifier could certainly be a help with this also.
When you get it all sorted out and like the tone, I'd sure love to hear some soundclips.
With respect, Tubenit
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I think these plate voltages are just too high. Although I have operated 6L6's around 400+VDC, 450 seems excessive.
They are not too high. Fender BF Super Reverb runs the 6L6GC' plates at 460dcv to 470dcv with NO problems.
And for sure the PI 12AX7 (B+3) is way too high.
No it's not way too high. BF SR B+ node C that feeds the PI is 450dcv. The plates of the 12AT7 have 230dcv on them.
Edit: I didn't see this is a K biased amp. Put in the rectifier tube, you should be good then.
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Since this is a cathode biased amp and it has the words "Tweed" and "Bluez" in the name, the original poster may prefer the sound of lower B voltages on the screens, PI, and preamp tubes. But that thought is me trying to predict what sound another builder would prefer :dontknow: I have my own biases. :icon_biggrin:
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Since this is a cathode biased amp and it has the words "Tweed" and "Bluez" in the name, the original poster may prefer the sound of lower B voltages on the screens, PI, and preamp tubes. But that thought is me trying to predict what sound another builder would prefer :dontknow: I have my own biases. :icon_biggrin:
This build was inspired by a spare 290CX and 1760J sitting on my bench for years, also by the fact that most of my builds have been fixed bias -- I wanted to do something different. I love the tone of my Dumble 124 clone, but came across Tubenit and DaGeeezer's Bluesmeister build -- which really grabbed me. K biased, dumble style OD -- this is just what I wanted. I was aware that the 290CX would likely be overkill for this project, but . . . that was my excuse for the build, and I figured I'd just deal with it.
I'd prefer the lower voltages just to explore what that sounds like.
So all of your observations and suggestions are much appreciated! I think this forum is awesome.
I'll probably try the 5D4 approach first and see what happens. I planned for and do have space for a GZ34 (or other) rectifier, and it seems likely I'll end up doing that, but will try the less invasive approach first. If I do replace the SS rectifier, I'll have to decrease the value of the reservoir capacitors.
Tubenit will definitely get his sound clips (though from a much less than stellar guitar player).