johngovan wrote: ↑Sat Jan 04, 2020 3:32 pm
This is interesting. Thank you for your suggestions and advices. I really appreciate it. As of now, what i have in hand is a PT with 250-0-250v with 50v bias tap and 6.3v filament tap. And OT with 8k primary impedance and a 4 ohm secondary. Okay. I'm on it. I will use 6v6 power amp stage. And try to learn biasing a fixed bias amp. I will try to search for step by step biasing. I watched a video on the YouTube of TubeDepot biasing a fender hot rod deluxe. Maybe it can help me in understanding how to bias a fixed bias amp
Ghetto method:
turn the bias pot until the plates start to glow red and then knock it back until they don't .....job done.......
johngovan wrote: ↑Sat Jan 04, 2020 3:32 pm
This is interesting. Thank you for your suggestions and advices. I really appreciate it. As of now, what i have in hand is a PT with 250-0-250v with 50v bias tap and 6.3v filament tap. And OT with 8k primary impedance and a 4 ohm secondary. Okay. I'm on it. I will use 6v6 power amp stage. And try to learn biasing a fixed bias amp. I will try to search for step by step biasing. I watched a video on the YouTube of TubeDepot biasing a fender hot rod deluxe. Maybe it can help me in understanding how to bias a fixed bias amp
Ghetto method:
turn the bias pot until the plates start to glow red and then knock it back until they don't .....job done.......
M
and then go buy some new tubes as you've cooked em
Okay. Upon reading and reading. I saw that 6L6 can make 26watts to 34watts of output power using 360v PT with cathode bias or fixed bias switch.. So I ordered 2x6L6GC so i can have more options of output for my project. So i can use that same tube when i will try to build a bigger 50watts 6L6 power amp.
rootz wrote: ↑Tue Dec 31, 2019 11:18 am
I think with the reverb set to 0 (or no reverb at all, like in your case) the loading of V2 should be different too. Keep in mind the anodes are tied together at that point in the original amp. Thus the plate resistor or the reverb mixer stage forms an AC path to ground via the power supply. Not really sure what the effect is though.
This actually didn't occur to me. Should one place another 100K plate to ground to simulate the loading of the other side (ie reverb mixer) of the anode?
I just tried and adding another 100k to ground is the way to go. Feels exactly like my other amp with reverb at zero
I don't really get this. A bit more reading (or consulting here) would have lead you to the conclusion that a pair of 6L6 tubes are not really suited here. The primary impedance of your output transformer (OT) does not provide what is needed for a pair of 6L6 at roughly 320VDC (250VAC wont get you 360VDC). Add to that, that your OT's primary of 8k suggests it's made for 2xEL84/6V6 so it will not have the capability to provide more than say 20W max, no matter what power tube you use.
You either need to read more or stick more to proven designs. By the latter I mean power amps with an 8k primary OT.
I need more data on the power transformer (PT). You say it's a 250-50-0-250 VAC. Is it a Hammond 370JX? If unknown, a reading of the resistance of the primaries would work to estimate the current capabilities, as does the weight and size of the transformer.
rootz wrote: ↑Tue Dec 31, 2019 11:18 am
I think with the reverb set to 0 (or no reverb at all, like in your case) the loading of V2 should be different too. Keep in mind the anodes are tied together at that point in the original amp. Thus the plate resistor or the reverb mixer stage forms an AC path to ground via the power supply. Not really sure what the effect is though.
This actually didn't occur to me. Should one place another 100K plate to ground to simulate the loading of the other side (ie reverb mixer) of the anode?
I just tried and adding another 100k to ground is the way to go. Feels exactly like my other amp with reverb at zero
Did you add a cap in series with that resistor? Otherwise you'd create a voltage divider and drag the plate voltage down to half...
Anyway, I worked out a power supply around a 250-50-0-250 VAC transformer. Could look like this. Enough bias range to get a pair of 6V6 biased properly (they would need approx. -26VDC). Flick the switch and add R7 to the circuit and you'd be set for EL84 tubes; bias voltage is lowered in the -14VDC range.
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Oh and I was thinking about the triode you have left if you build this circuit. You could add a tremolo like in the Fender Princeton or this idea from Merlin. Simple bias shift tremolo, you'd only need some cheap parts, a switch and two pots to make this. Might be an idea.
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This actually didn't occur to me. Should one place another 100K plate to ground to simulate the loading of the other side (ie reverb mixer) of the anode?
I just tried and adding another 100k to ground is the way to go. Feels exactly like my other amp with reverb at zero
Did you add a cap in series with that resistor? Otherwise you'd create a voltage divider and drag the plate voltage down to half...
Thanks for the reply rootz, but I didn’t use any capacitador.
I took the approach of the FET and replicated to this situation. Normally on the Dumble amps the tonestack and the FET are fed by the same anode supply. Assuming a low plate amp, the tonestack would have 2x 100k plates (one for each triode) and the FET would have either 150k or 220k.
Folks here taught me that, on amps without a FET one should add the 150k of the FET to ground, to simulate having it in the circuit.
I thought that, in an amp with only 1x 100k plate in order to simulate the other plate, the FET approach (ie 100k to ground) would work.
But it seems like not a good approach what I did? Any suggestions on how to simulate the 100k of the other triode feeding the same anode?
rootz wrote: ↑Sun Jan 05, 2020 4:57 pm
Oh and I was thinking about the triode you have left if you build this circuit. You could add a tremolo like in the Fender Princeton or this idea from Merlin. Simple bias shift tremolo, you'd only need some cheap parts, a switch and two pots to make this. Might be an idea.
Princeton bias tremolo is the nuts my favourite trem if I could have worked that into my JM sig I'd have been very happy.
I just tried and adding another 100k to ground is the way to go. Feels exactly like my other amp with reverb at zero
Did you add a cap in series with that resistor? Otherwise you'd create a voltage divider and drag the plate voltage down to half...
Thanks for the reply rootz, but I didn’t use any capacitador.
I took the approach of the FET and replicated to this situation. Normally on the Dumble amps the tonestack and the FET are fed by the same anode supply. Assuming a low plate amp, the tonestack would have 2x 100k plates (one for each triode) and the FET would have either 150k or 220k.
Folks here taught me that, on amps without a FET one should add the 150k of the FET to ground, to simulate having it in the circuit.
I thought that, in an amp with only 1x 100k plate in order to simulate the other plate, the FET approach (ie 100k to ground) would work.
But it seems like not a good approach what I did? Any suggestions on how to simulate the 100k of the other triode feeding the same anode?
Was it in the same place as the R17 in my schematic above?
I'm not trying to replicate something like the FET. The FET loads down the power supply at the end of the chain, so you emulate this with a resistor. Note: it loads the DC!
The resistor and cap I propose emulate the other part of the dry/wet mixer, the last triode of the reverb. There is no actual DC path to ground if I see it correctly, just an AC path. Well actually, there is a tiny pit of current flowing from the one to the other triode in the mixer, but that's micro amperes. Thus, according to my logic, only loading for AC should be accounted for.
rootz wrote: ↑Sun Jan 05, 2020 4:57 pm
Oh and I was thinking about the triode you have left if you build this circuit. You could add a tremolo like in the Fender Princeton or this idea from Merlin. Simple bias shift tremolo, you'd only need some cheap parts, a switch and two pots to make this. Might be an idea.
Princeton bias tremolo is the nuts my favourite trem if I could have worked that into my JM sig I'd have been very happy.
perhaps I should build a 50/100w princeton reverb
M
I'm thinking about a 2nd gen ODS with one tube reverb and bias shift trem (oscillator + cathode follower). That would be fun!
However, next thing I want to find out is how to build a HRM with separate masters AFTER the reverb (to get enough signal to the reverb driver, think overdriving the tank like in old Fenders). Cracks my brain though...
Did you add a cap in series with that resistor? Otherwise you'd create a voltage divider and drag the plate voltage down to half...
Thanks for the reply rootz, but I didn’t use any capacitador.
I took the approach of the FET and replicated to this situation. Normally on the Dumble amps the tonestack and the FET are fed by the same anode supply. Assuming a low plate amp, the tonestack would have 2x 100k plates (one for each triode) and the FET would have either 150k or 220k.
Folks here taught me that, on amps without a FET one should add the 150k of the FET to ground, to simulate having it in the circuit.
I thought that, in an amp with only 1x 100k plate in order to simulate the other plate, the FET approach (ie 100k to ground) would work.
But it seems like not a good approach what I did? Any suggestions on how to simulate the 100k of the other triode feeding the same anode?
Was it in the same place as the R17 in my schematic above?
I'm not trying to replicate something like the FET. The FET loads down the power supply at the end of the chain, so you emulate this with a resistor. Note: it loads the DC!
The resistor and cap I propose emulate the other part of the dry/wet mixer, the last triode of the reverb. There is no actual DC path to ground if I see it correctly, just an AC path. Well actually, there is a tiny pit of current flowing from the one to the other triode in the mixer, but that's micro amperes. Thus, according to my logic, only loading for AC should be accounted for.
It was actually at R27 where I added the second 100k to ground but after the recovery stage and decoupling cap my circuit goes straight to the master..but I understand your point...will remove it!
rootz wrote: ↑Sun Jan 05, 2020 4:57 pm
Oh and I was thinking about the triode you have left if you build this circuit. You could add a tremolo like in the Fender Princeton or this idea from Merlin. Simple bias shift tremolo, you'd only need some cheap parts, a switch and two pots to make this. Might be an idea.
Princeton bias tremolo is the nuts my favourite trem if I could have worked that into my JM sig I'd have been very happy.
perhaps I should build a 50/100w princeton reverb
M
I'm thinking about a 2nd gen ODS with one tube reverb and bias shift trem (oscillator + cathode follower). That would be fun!
However, next thing I want to find out is how to build a HRM with separate masters AFTER the reverb (to get enough signal to the reverb driver, think overdriving the tank like in old Fenders). Cracks my brain though...
The thing about bias trem is that depending the levels you run your amp sooner or later you are prone to blow the PT. Schofield did that a few times on his two rock sig which has a bias trem
