Removing input pot in tube amplifier
- Thread starter robnec
- Start date
s-petersen
Scott
it would probably work best that way, If there is already some other resistors between the the grid and ground it may not need it.
Do you have a schematic?
Do you have a schematic?
s-petersen
Scott
It is a 7199 IIRC, and the performance is poor with the stock input system. I have to go shopping, when I get back, I 'll write some more
s-petersen
Scott
Remove the 250k pot, replace the 33k resistor with a 100k resistor, I think that will make it work properly
Schematic
https://www.dropbox.com/s/j8feyozxm984hos/conn 720 amp.JPG?dl=0
Schematic
https://www.dropbox.com/s/j8feyozxm984hos/conn 720 amp.JPG?dl=0
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s-petersen
Scott
Yes, but the small transformer may not sound as good as the 7027 ones, I have not thoroughly tested the small one, but I was was not getting great results for the first tests I did.
for some reason there seems to be a problem posting pictures, I can't post the schematic in high res, as before I could
for some reason there seems to be a problem posting pictures, I can't post the schematic in high res, as before I could
Hello. I have added 100k resistor to ground and left alone 33k at the grid.it seems to work ok. Being inside I also redid bias supply and now I can adjust each 7027 tube separately. Bias voltage looks little high at -33 volts with 440v. I have to get smaller value resistor, maybe 200ohms. Thank you all for help. Robert.
s-petersen
Scott
So how did you make adjustable bias? The amp has no negative supply, Did you change the 270k from the grids to ground to an adjustable resistor?
s-petersen
Scott
I left them as cathode biased, even in the Mullard conversion. You can add a small transformer, rectifier,a cap, some pots. to build a separate negative supply, and replace the 250 ohm resistors with 4 separate 10 ohm resistors to ground from each tube, using them to calculate current in each tube.
I am not sure if what you did will work properly under load. Dave G, I'm sure would know better than I, I'm certainly not an expert.
I am not sure if what you did will work properly under load. Dave G, I'm sure would know better than I, I'm certainly not an expert.
primosounds
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You made the power tube adjustable cathode bias. It is not a big deal other than maybe you need to be careful of the current capacity of your pot. I would guess around 2 watts to be safe. You can now adjust your power tube bias resistor from 250 to 272 ohms. If 250 was the original/stock bias resistor you can now run the 7027 "cooler" by dialing in more resistance. But, practically, your pot is not going to affect things too much. It would be more "adjustable" to have the resistor at 200 ohms and use a 100 ohm pot to give you a 200 to 300 ohm range. What was the voltage you measured at the cathode across the cathode resistors? Since this is your amp making this mod is up to you. What you accomplished is not really fixed bias which uses a negative voltage on the grid to establish the idling point of the tube. But, essentially the same thing in that you adjust the current flow through the cathode instead. I am certain i have seen this in some schematic or two.
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Hello. They are 3 watts. Do I need these adjustments? Amplifier is working fine but if this "feature" does nothing sound wise I will remove them all. Simpler is better. They are liquidating Radio Shacks in the city so at over 50%off I bought a lot of different parts. Most not necessarily needed but who knows... Robert.
Ah - just to confrim; glad that came up. The series 33K in a triode-input stage of a NFB amplifier will have something to do with NFB h.f. stability; should be kept in. (And perhaps over-cautious; but a series input capacitor might not be amiss. Yes! One expects a pre-amplifier output to be at earth d.c. potemtial - but these days anything is possible .... )
s-petersen
Scott
The input stage is pentode IIRC
