Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on September 09, 2021, 03:50:47 am
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Hey guys
I'm doing a 6G3 build, but added a BF Reverb circuit. Assuming there are too many triodes on the D-node I decided to add another node (E). Since node D is filtered with an 8uf electrolytic cap I thought about doing the same for node E. Between those nodes I added a 1k dropping resistor. Is this ok what I'm doing?
Another question. Is it ok to put the reverb pot next to the output RCA of the reverb, or should there be more space in between those?
also, I drew the .022uf coupling cap after the plate of V2a. But I don't think it's necessary, or is it?
thx
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Here is another example of another way to add a node to B+. Not necessarily better just different.
Alot of it depends on what you're needing the voltage at a particular node to be. So, the additional node doesn't necessarily have to be at the end of the B+ line (so to speak).
With respect, Tubenit
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As it is, the 2nd cascaded recovery stage will cause problems.
And it's poor practice to share cathodes on cascading stages, creating problems for the future when the bypass cap develops high ESR.
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As it is, the 2nd cascaded recovery stage will cause problems.
And it's poor practice to share cathodes on cascading stages, creating problems for the future when the bypass cap develops high ESR.
That's a shame... my first drawing had those K's separated. Until I saw that the docs I was using had them shared. I should have let it like that...
Thanks for the concern.. I will add some turrets and separate them.
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What's the intended purpose of the two diodes near the cap can on the layout? The reality is the amp cannot work with those diodes.
Bias circuit ac feed (brown wire) will not work as drawn. Brown wire must be connected directly to one of the PT red wires.
PT HV winding CT is missing. Probably just an oversight?
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As it is, the 2nd cascaded recovery stage will cause problems.
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The 470k was probably a mixer resistor. It makes no sense to have it as a grid stopper on the 2nd recovery stage. And the low output impedance of the 2nd stage (about 40k) will cut the dry signal level significantly.
It would all work a lot better if the 470k was returned to its function as a mixer resistor, moved to the output of the second stage.
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What's the intended purpose of the two diodes near the cap can on the layout? The reality is the amp cannot work with those diodes.
Bias circuit ac feed (brown wire) will not work as drawn. Brown wire must be connected directly to one of the PT red wires.
PT HV winding CT is missing. Probably just an oversight?
Thx Steve. I will look into those remarks.the diodes are optional for dropping B+ if necessary. And they should be zeners..misdrew that. Sorry for the confusion
Btw ... I didn't draw any CT yet. Nor the output to speaker wiring...will complete the layout when revising
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As it is, the 2nd cascaded recovery stage will cause problems.
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The 470k was probably a mixer resistor. It makes no sense to have it as a grid stopper on the 2nd recovery stage. And the low output impedance of the 2nd stage (about 40k) will cut the dry signal level significantly.
It would all work a lot better if the 470k was returned to its function as a mixer resistor, moved to the output of the second stage.
Maybe ditch the 470k then? There's no mixing to be done..I think?
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Maybe ditch the 470k then? There's no mixing to be done..I think?
The wet reverb signal needs mixing back into main signal path.
100k feeds the dry signal to the next stage.
As drawn, the 40k output impedance of the 2nd recovery stage will cause the reverb signal to dominate the mix.
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Maybe ditch the 470k then? There's no mixing to be done..I think?
The wet reverb signal needs mixing back into main signal path.
100k feeds the dry signal to the next stage.
As drawn, the 40k output impedance of the 2nd recovery stage will cause the reverb signal to dominate the mix.
I might be going to redraw the reverb to the circuit in my imperial clone. Great reverb, that one!
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I revised the layout. Not doing reverb anymore... going to build a stand alone unit.
My guess the 6G3 is best without the on board reverb.
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IF you do NOT need surf style reverb, .................... I think this would work fine for you. I am currently using this on my Carolina Overdrive Special and have the reverb pot set at about "3" and it's all the reverb I need. Using a 12DW7 as your one tube reverb works well also. OR a 12AX7 or 12AT7.
With respect, Tubenit
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IF you do NOT need surf style reverb, .................... I think this would work fine for you. I am currently using this on my Carolina Overdrive Special and have the reverb pot set at about "3" and it's all the reverb I need. Using a 12DW7 as your one tube reverb works well also. OR a 12AX7 or 12AT7.
With respect, Tubenit
Thanks.. I already have a few amps with the 1-tube reverb. But for the music I play it's not the right reverb.. it's strong enough, just not full enough. Dunno, it just lacks.
I'm going to think about it some more, but the blackface reverb suits me the best.
Input side of reverb:
I also find it hard to understand where to implement it. Normally I'd say coming off the 100k plate load resistor of the stage before the PI. But the 6G3 has a series resitor there 15k and 100k and goes to the PI from the conjuction of those resistors. It confuses me. I don't know if the amp will behave weird coming from the 15k instead of the 100k at the tap for the reverb input.
Recovery side of reverb:
If I understand correctly I should add a mixing resistor between the plate of the recovery side and the grid of the cascaded amplification stage? I see values like 3.3M of 1M or 470k... which is it? And how does that value affect things?
MV confusion too...
I drew the LARMAR on the layout. But I have read that this one messes with the bias and I should be using the version from Bruce Collins. Is that a fact? Should I rather be using that one? (added caps and resitors)
http://el34world.com/Forum/index.php?topic=15026.0
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Input side - I would not anticipate any issue with the reverb there regarding the 15k. Why would that matter? That plate sees a 115k resistance since there is a
split between the 15k and 100k. My understanding is the split allows for a "cleaner" less break up tone. I used that approach on my Hoso56 The Minimalist
Recovery side - Fender uses a 100k reverb pot and a 3.3M. The one tube reverb uses a 1M reverb pot and 150k-220k. OR some people use a 2.2M reverb pot.
It is isolated well enough using a 1M reverb pot vs. a 100k reverb pot.
However, since you are clear with yourself on one tube vs. Fender style ............. I would certainly stick with the Fender style on this amp as your preference. Fender reverb sounds great. Hard to go wrong with it.
With respect, Tubenit
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I revised the layout. Not doing reverb anymore...
:thumbsup:
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I revised the layout. Not doing reverb anymore...
:thumbsup:
Steve..just to be sure:
A. 👍A 6G3 is better without reverb?
B. 👍layout looks way better now?
C. 👍I shouldn't do the reverb because I'm not capable?
D. 👍something else?
I know you can be sarcastic.. so sorry for me being confused..😉
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> 8uf electrolytic cap ...... a 1k dropping resistor. Is this ok
Why so little resistance? Are Ohms expensive?
There's probably less than 3mA here. You can afford a LOT more drop than 3V. And the filtering gets better directly.
And 8uFd 1K is awful close to the audio band, 20Hz. If you fear motorboating, this is asking for trouble.
Also the reverb recovery probably needs to be filtered at least as well as the guitar input stage.
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> 8uf electrolytic cap ...... a 1k dropping resistor. Is this ok
Why so little resistance? Are Ohms expensive?
There's probably less than 3mA here. You can afford a LOT more drop than 3V. And the filtering gets better directly.
And 8uFd 1K is awful close to the audio band, 20Hz. If you fear motorboating, this is asking for trouble.
Also the reverb recovery probably needs to be filtered at least as well as the guitar input stage.
Agreed...I just put in a resistor , not to drop voltage, but to separate filter caps.
The original 6G3 has 8uf filter for the preamp too, but with 27k... is it better to raise that value for the reverb recovery to 27k too?
Or maybe I should split the D node into 2 separate strings of 27k and 8uf? Problem is I already made the turretboard and have to be creatieve to do so..
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I thought you were not doing reverb anymore? If so, you don't need the extra B+ node.
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I thought you were not doing reverb anymore? If so, you don't need the extra B+ node.
Maybe I got reinspired 😄
I have all the stuff for the verb, the chassis is already changed for 5 preamp tubes... so maybe, Yes.
I can always add a switch to take the reverb out of the circuit...
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> to separate filter caps.
When I have to separate my dogs, I don't use a handkerchief.
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> to separate filter caps.
When I have to separate my dogs, I don't use a handkerchief.
Pardon me for not getting that. Might be for my Belgian nationality..
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Ok, I added the complete BF reverb as I can see it to the schematic.
This should be correct?
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The original blackface reverb circuit mixing stage also had a coupling cap and a 50k pot (from the trem circuit) that prevent really ugly distortion. I include those when I graft this to a non reverb circuit. Hoffman uses a 47k resistor to ground instead of the trem pot. Look at his "one channel AB763". link Tube Amp Schematics at the bottom of this page.
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> to separate filter caps.
When I have to separate my dogs, I don't use a handkerchief.
Pardon me for not getting that. Might be for my Belgian nationality..
I think the point is that a 1k dropper with an 8uF cap is too small to do a good job at decoupling one HT node from another.
ie increase the value of the 1k, the 8uF, or both, in order to a more achieve effective decoupling.
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Ok, I started building it. I bit the bullet and I'm not doing reverb on board. Next I'm building a stand alone reverb for that. I wanted one anyway...
The normal channel is a plexi like preamp, inspired on the Cutthroat Audio Down Brownie.