Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ernie_jr on September 08, 2013, 09:56:56 am
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I am planning to build a hybrid ampeg amp. When looking at all the schematics ampeg uses the 7199 alot. I have several 6u8s that I want to use. My question is, if you look at the PI in the B26, B 18n and the V3 they all use the 7199 but have different components and set ups. Which of the 3 would be the preffered? The V3 is the simpilest, but it does not have negative feedback.
Thanks for your input,
Ernie
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No opinion.
Don't really see a dime's-worth of difference.
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Thanks PRR, will just pick one and go with it.
Ernie
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The V3 has bootstrapped bias on the PI as compared to fixed bias on the B18. I like the bootstrapped bias but I don't know why. I'm not familiar with the B26?
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Sorry,
B25, what is a bootstrap bias?
Ernie
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In a tube circuit such as a cathode follower or PI you will have a large positive voltage on the cathode. You will need to also put a slightly lower large positive voltage on the grid for proper biasing. (Grid needs to be slightly negative with respect to cathode). An easy way to do this is to use a voltage divider between B+ and ground to put a "fixed" voltage on the grid. Or you can connect the grid resistor to the cathode circuit (bootstrap) to get a positive voltage on the grid.
An even easier/simpler way to get a positive voltage on the grid is to dc couple the grid to the plate of the previous stage. Look at the Sunn amps which use a 6AN8 (very similar to the 7199).
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Thanks Sluckey,
Have built several SUNNs, They are simple builds, I will compare the 2 PI's.
Thanks,
Ernie
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I am planning to build a hybrid ampeg amp. When looking at all the schematics ampeg uses the 7199 alot. I have several 6u8s that I want to use. My question is, if you look at the PI in the B26, B 18n and the V3 they all use the 7199 but have different components and set ups. Which of the 3 would be the preffered? The V3 is the simpilest, but it does not have negative feedback.
coupling caps between the pentode and the PI as well as PI->7027A's all designed for super low bass roll-off (nearly zero), but even the Gemini guitars amps were designed with similar roll-off. An exception might be that B18N, but its still low. nonetheless, whether you can source quality .3uf's might make you sway one way or the other.
Cathode by-pass cap arrangement varies a bit, and the B18N doesn't have a cathode bypass cap. The screen by-pass cap varies slightly (.5 vs .1), I'm not familiar enough with pentode gain stages to know if .5 vs .1 in that spot happens to be any kind of BFD. maybe all by-pass cap variations are the result of some fine-tuning for bass response with a particular speaker design?
Like you pointed out, NFB varies, and some Ampegs use an 16ohm tap for NFB while others use an 8ohm tap (important if you are grafting schematics together for hybrid designs).
B25 and V3, not too common! how'd you pick those?
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Thanks terminalgs,
Picked these by just looking at the schematics. I have built several SUNNs and a hybrid Ampeg B15. As I stated, I have several 6u8s to use (similar to 7199, diff pin out) so wanted something with the pentode/triode PI set up that would be diff from a SUNN.
Ernie