You'll find the zeners have a positive temperature coefficient - bias goes more negative as the zeners heat up. Some forward biased diodes in series can compensate for this - it'll probably take at least a half dozen (each giving -2 mV per degree C).will
Alot of 6550 brands also drift.....the B version is notoriously heater volts sensitive. My EH KT90's are rock steady.
I changed R29 and R36 to 33k and R28, R35 to 10k. I also changed pot R47 to 10k. This gives an impedance of approximatley 43k to ground from the grids.
I tested the circuit and one tube was rock solid stable in bias current but the other tube drifted like crazy; up or down. I then switched the tubes and the same one was drifting. I guess I have a bad tube. Since a few people have heard that the EH KT88 are sometimes known to drift I guess this is plausible. I will get another matched pair and try them out.
I tested the circuit and one tube was rock solid stable in bias current but the other tube drifted like crazy; up or down. I then switched the tubes and the same one was drifting. I guess I have a bad tube. Since a few people have heard that the EH KT88 are sometimes known to drift I guess this is plausible. I will get another matched pair and try them out.
I know that 6L6 STR's are a different animal but in a push pull guitar amp I built using a quad of 6L6GC power tubes, the first quad I got was Tung Sol 6L6.
They were supposedly a matched quad and did bias up properly when new.
My amp has a bias pot for each side or each pair of tubes.
After a few weeks of playing I noticed that the tubes had drifted quite a bit.
On one pair there was a 10ma difference. I usually don't sweat up to about a 5ma difference on a guitar amp, in fact it can make for pleasant harmonic overtones but over 10 ma I draw the line.
I switched them around trying to find the combination that would work.
But the problem followed a couple of the tubes.
Finally I ordered a quad of TAD 6L6 STR tubes from Doug's Tubes.
He claims that he burns in all of his tubes for at least 24 hours if not longer.
I think this really helps because this quad has stayed pretty well balanced with only about a 4 ma difference between two tubes at the most now.
Even if they claim they match the tubes, if they are not burned in to where they settle in, I think there is every chance that they will drift quite a bit after break in.
They were supposedly a matched quad and did bias up properly when new.
My amp has a bias pot for each side or each pair of tubes.
After a few weeks of playing I noticed that the tubes had drifted quite a bit.
On one pair there was a 10ma difference. I usually don't sweat up to about a 5ma difference on a guitar amp, in fact it can make for pleasant harmonic overtones but over 10 ma I draw the line.
I switched them around trying to find the combination that would work.
But the problem followed a couple of the tubes.
Finally I ordered a quad of TAD 6L6 STR tubes from Doug's Tubes.
He claims that he burns in all of his tubes for at least 24 hours if not longer.
I think this really helps because this quad has stayed pretty well balanced with only about a 4 ma difference between two tubes at the most now.
Even if they claim they match the tubes, if they are not burned in to where they settle in, I think there is every chance that they will drift quite a bit after break in.
I've only done the separate pot for each tube method, but I would imagine that using a balance pot and a common adjusting pot between pairs may be easier to adjust.
When adjusting separate pots on each tube, changing the bias on one changes the other, so you end up going back and forth a few times to get everything happy. With the balance pot, it would seem like you could get the bias close to where you want it then then just balance the tubes.....then again, this may require going back and forth between the pots a few times.....
For certain the Grid resistors are too large: i mostly use 50K with fixed bias, but you need lots of drive.
Also worth checking on the Oscilloscope for oscillation.
Russian KT88's are more like 6550's they work best with 50K. Test have shown they are 6550's not anything like GEC KT88"s. The EH 6550's /KT88's are very good tubes!
Phil
Also worth checking on the Oscilloscope for oscillation.
Russian KT88's are more like 6550's they work best with 50K. Test have shown they are 6550's not anything like GEC KT88"s. The EH 6550's /KT88's are very good tubes!
Phil
I have reduced the grid resistors to less than 50k and everything is fine now. I had one bad tube out of four. I have read reviews about the EH KT88's and there were no complaints so I bought them. I am new to the wonderful world of tubes so I have lots to learn.
I am not to worried about loading the drivers too much since they have an output impedance around a 100 Ohms.
Ah...that's the cold water hoax...Unfortunately with tubes used in basic R/C configuration with the anode as sig output and the following stage with a loading grid bias resistor to ground, the graph relating to distortion and output impedance is quite noticeable without a serious signal level change (due to the lower AC internal resistance of the tube) at the output, but as the shunt loading goes up, so distortion will rise. The circuit has to be designed to grind the distortion out and many appallingly designed amps often use a single loop of heavy global feedback from 25-35dB to sort this problem out. I shan't get involved with this sonically, each person has differing views on this.
A cathode follower and Circlotron closed loop types offer the best performance.- Generally valves love high impedance and ye more output loading will in most cases create a distortion rise behind ones back and that's where the choice of tubes for driver stages becomes important.
There is a commercial circuit that uses an ECC83 for driving an KT88...some designers oft for such heavily slewed circuits as to make the circuit stable with such a crummy bandwidth but for that little tube it's a slaughter house policy to deal with Miller effect.
Keep at it !!
richy
Last edited:
- Status
- Not open for further replies.