Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dusty712 on April 04, 2025, 10:29:03 am
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I'm starting a build of a Sunn Model T clone, using this schematic (http://www.prowessamplifiers.com/schematics/post/sunn_model_t.pdf) and using Ceriatone's layout as a base (https://ceriatone.com/wp-content/uploads/2020/03/Model-T-15-Dec-2019.jpg). I picked up their turret boards and components (minus transformers) just to make it easier for me to get started.
One thing I wanted to add is individual bias pots to each tube and I was hoping to get some advice on how to modify the original schematic to support this. If there are some useful links that would be helpful to read through to understand this better, that would appreciated as well.
Thank you.
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Add 3 more bias pot's circuit (. R44,R45, C16 ) where the original one is wire at the bias rectifier diode CR1. ;
Separate each 100 k resistor where you see the bias negative voltage, one for each tube . You need 2 more 100K
You need also 2 coupling cap like C12, C13
IMO it is too much work and parts for nothing, no amp need a separate bias pot.
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Google
https://atrad-audio.co.nz/index.php/2018/12/17/pp-fixed-bias-circuit-design-and-calculator/
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Rather than have four bias pots may I suggest only two pots? One pot for each parallel pair. You may get some ideas from this modified Sceptre (https://sluckeyamps.com/sunn/sunn.htm). It's only two tubes but the circuit would be the same for four tubes. Scroll to the bottom of the page for the mod and schematic...
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Thank you all for your help. I'll dig through the information shared, but so far I'm liking the idea of the simpler two pot approach. The link to the modded Sceptre helps and it doesn't look too complicated to fit.
I'm just getting started into this and there's a lot to learn, so I appreciate the advice that using individual bias pots isn't really that beneficial.
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Plan keeps Sunn scheme, just doubles it. You get best source isolation with less control interaction with two independant supplies.
--Pete
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Plan keeps Sunn scheme, just doubles it. You get best source isolation with less control interaction with two independant supplies.
--Pete
1st link do not work
Second link picture ; easiest with picture here.
There are no need for two IN4004
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Plan keeps Sunn scheme, just doubles it. You get best source isolation with less control interaction with two independant supplies.
--Pete
There are no need for two IN4004
For maximum isolation, yes, you most certainly do.
--Pete
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Plan keeps Sunn scheme, just doubles it. You get best source isolation with less control interaction with two independant supplies.
--Pete
There are no need for two IN4004
For maximum isolation, yes, you most certainly do.
--Pete
Why the need for more isolation ?
More isolation ; Why not a separate transformer ?
I'm completely wrong.
You absolutely need a diode for each of the bias adjustment circuits (pot, resistors, etc.), as DummyLoad mentioned.
If these circuits are not separated by a diode, the pot setting of one circuit will influence the setting/voltage, and therefore the bias, of the other circuit.
An example with two bias circuits: by setting one lamp cooler, the other lamp will automatically be set warmer.
With a risk of red plating
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1st link do not work
Sure it does. You just need the proper program. Download ExpressSCH.
https://el34world.com/Forum/index.php?topic=590.msg1622#msg1622
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I used five pots in my scratch build. One is bias "master" and it has a test point for bias voltage. Four other pots are 10K center-clicks. All four tubes have 1 ohm resistors and individual test points. Results were very useful. And easy to solder... Planning was the harder part. I'll update the project blog today.
https://el34world.com/Forum/index.php?topic=32536.0
Look at the left side of Proj100pwr.pdf, it shows how this is done. Those four test points at tubes are missing, but they are soldered to the cathodes.
If anyone decides to do something like this, remember to check mA's at least twice. I found out it is a bit like tuning a Floyd Rose, when using heavily mismatched tubes.
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I'm completely wrong.
You absolutely need a diode for each of the bias adjustment circuits (pot, resistors, etc.), as DummyLoad mentioned.
If these circuits are not separated by a diode, the pot setting of one circuit will influence the setting/voltage, and therefore the bias, of the other circuit.
An example with two bias circuits: by setting one lamp cooler, the other lamp will automatically be set warmer.
With a risk of red plating
DummyLoad, please excuse me
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No worries Latole. :smiley:
Fusing the cathode of each KT88 can help alleviate destruction of the transformers in event of a catastrophic tube failure.
--Pete
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No worries Latole. :smiley:
Fusing the cathode of each KT88 can help alleviate destruction of the transformers in event of a catastrophic tube failure.
--Pete
It's a great idea, but why haven't you ever seen it at an amp manufacturer?
On the other hand, a fuse in the high-voltage circuit, often seen, could provide sufficient protection?
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No worries Latole. :smiley:
Fusing the cathode of each KT88 can help alleviate destruction of the transformers in event of a catastrophic tube failure.
--Pete
Or use fuseable cathode (and screen) resistors
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No worries Latole. :smiley:
Fusing the cathode of each KT88 can help alleviate destruction of the transformers in event of a catastrophic tube failure.
--Pete
It's a great idea, but why haven't you ever seen it at an amp manufacturer?
On the other hand, a fuse in the high-voltage circuit, often seen, could provide sufficient protection?
I have - HiFi stuff mostly and they usually use a fusible resistor. Newer guitar amps fuse each secondary, actually, to pass UL and other certifications, most certification agencies require that every secondary be fused. If I were to clone another Model-T, I'd fuse each of the tubes with a 1/2A fast-acting fuse. On that last Model-T I cloned, I got lazy and simply installed an HT fuse on the HV CT.
I don't trust modern production tubes, the QC these days is nothing close to what it was when tubes ruled the world.
--Pete
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[
I have - HiFi stuff mostly and they usually use a fusible resistor. Newer guitar amps fuse each secondary, actually, to pass UL and other certifications, most certification agencies require that every secondary be fused. If I were to clone another Model-T, I'd fuse each of the tubes with a 1/2A fast-acting fuse. On that last Model-T I cloned, I got lazy and simply installed an HT fuse on the HV CT.
I don't trust modern production tubes, the QC these days is nothing close to what it was when tubes ruled the world.
--Pete
I agree with you
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Update,
I've finally found what I think is an excellent circuit for a double bias adjustment. And with a single diode.
It's from a book by Kevin O'Connor. The Ultimate Tone 2 page 3-16 ( see screenshot) .
It's similar to the circuit I've already mentioned, but which doesn't work very well. O'Connor's works perfectly in my amp.
I connected to a 360 Vac output of the transformer. I added a 100k resistor* to lower the voltage by acting as a voltage divider with R1. I get 76 Vac before the diode.
* Arrow on my sketch.
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From K. O'Connor TUT book, don't remember which book. Then you don't need C3/C4 in the above schematic.
Has 22K safety R's across the pots so if the wiper fails, you still have -bias.
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There are several types of bias circuits.
A bias circuit always needs filter cap(s).
They can be placed in different places.
Your circuits are incomplete, missing an important part.
Too bad you don't have the exact reference, as I have all K. O'Connor's books.
I'm referring to previous answers where it was mentioned that a rectifier circuit had to be used for each potentiometer.
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What I posted shows -bias vdc in. The cap or caps would be in there.
No need to show that portion, it's showing what needs to be shown for a dual bias or a bias balance circuit.
And the circuit you posted is from his 2nd book, later he updated the circuit to include safety R in case the bias pot's wiper fails. Very important feature and very easily down.
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I agree that the 22 k connected to the pot slider is a safety feature.
On the other hand, I've never seen or heard of anyone having experienced the problem of a faulty wiper contact .
And as far as tube and circuit protection is concerned, we could mention a lot of good ideas.
But where to stop?
Are you sure your drawing is by K.O'Connor?
His drawings are mostly freehand, whereas yours was done with a guide/stencil.
That doesn't change the fact that it's a good circuit.
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This circuit is also discussed by Gar Gillies ( Garnet amps ) in his book “The How and Why of Guitar Tubes Amps as gar see it ” on page 32.
I quote him ; "this is a fine, accurate system of biasing......"
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What is your problem? I posted a bias circuit and you had to mouth off about it.
I've been watching you on all your posts for a long time, now you want to come after me. You better just back off now. This forum would be just fine without you, in fact it might be better.
I don't care what you think. Doesn't matter if you've never seen a bias pot go bad. I didn't quote you, I didn't put your name in my post. The circuit is fine, you don't need to judge it.
Your barking up the wrong tree and I've had about enough of you. I'm not going to argue with you, so stop it.
You've had a bad attitude since you 1st got here. You often write unnecessary reply's to people that are harsh and on top of that your wrong many times. You need to back off and write your replies in a softer tone.
I drew that from a TUT book, I have all his books, I drew it with PC Express, it was very popular on this forum before you got here. I've been posting that drawing here for YEARS.
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This circuit is also discussed by Gar Gillies ( Garnet amps ) in his book “The How and Why of Guitar Tubes Amps as gar see it ” on page 32.
I quote him ; "this is a fine, accurate system of biasing......"
I didn't say it wasn't.
All I said was if you use the O'Connor's later bias you won't need those 2 extra caps.
And you got all bent out of shape.
He wrote those books over a good number of years and he added things along the way as he grew in preference to doing things.
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Cool down guys.
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I just wanted to follow up on this and thank DummyLoad's advice and schematic. It made it really easy to implement and I just finished it off today. All works great, as expected so thank you again. I've only tested the bias circuit on its own, no tubes connected yet and I'm getting 44V - 64V bias range out of it.
Thank you as well to other members for their advice and for posting Kevin's TUT book schematics. I'm using his TUT3 book to build this amp, so I'll be taking a look at those as well later to see how he designed it.
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Happy to help and that it worked out for you. Please keep us apprised on progress.
Respectfully,
--Pete
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Happy to help and that it worked out for you. Please keep us apprised on progress.
Respectfully,
--Pete
Hey Pete, thank you again for your help and I just wanted to give you an update on my build, which I finally put together (mostly) and was able to power it on today. Being my first amp build I was nervous, but all went really well minus a couple of small issues that I was able to sort out. Sound wise, I'm quite happy with it and from the limited amount of time I had with it I think it sounds quite good (especially given the old tubes I'm using during the trial). The slightest of hum if I get up close to the speaker, but barely audible otherwise. It's almost there, just need to tidy up the wiring a bit, mount the chokes and box it up.
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Nice Work Dusty.
Just one (silly) question :w2:
Is that a dry joint on the P/S diode?
Regards
Mirek
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Nice Work Dusty.
Just one (silly) question :w2:
Is that a dry joint on the P/S diode?
Regards
Mirek
Thank you Mirek, and appreciate you looking it over. It's not a silly question, that diode is connected to the diode on its left, I just had its leg go under the board and connect it there. I didn't see a need to fill in that eyelet in this case, but I could be wrong so any feedback is always appreciated.