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In Reply to: RE: Here you go posted by Michael Samra on June 25, 2012 at 02:35:07
Look at the schematic. How did the designers choose to deal with g3 and the cathode? Why did they do what they did? Does a 6L6 or KT77 allow this circuit to operate as designed? Just because the thing operates without blowing up doesn't mean that it operates as it should. Web references from persons unknown changes nothing in this regard. You could use automatic transmission oil in your car's engine and it would probably run okay but that doesn't mean you should. Go ahead and roll 6L6s into your 8B and sure, it will probably work. You may even like it. But it ain't an 8B any more.
Follow Ups:
you will see there are several scenarios as far plate load operating points go.We learn new things everyday.The mids in an EL34 are very nice..The extremes are cut off even in the 8 by comparison when I use the EL34.Also,my xf2s and Xf1s are mushy by comparison to the KT77 and the KT66 as well the RFT el34s..The metal EL34s are ok when you bias them hotter.
As I have said,nobody has to quit using the EL34..I have over 200 el34s of my own..Its just fun to roll and experiment.Why do people soup up their cars?Its fun and its a nice effect.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
"Does a 6L6 or KT77 allow this circuit to operate as designed?"
I know what you mean, but...
... But if it operates in a way that pleases the ear of the listener - and it doesn't blow up - who's to say they are wrong?
...That tells me they don't care for an 8B. So why not get an amp that's designed for tube rolling? If I'm not mistaken, Roger Modjeski makes a point that some of his designs can take all kinds of output tubes with minor only adjustments. There are others. Bottom line is that if one owns an 8B and likes 6L6 types in it, fine with me. What I do find bothersome is when someone makes a tube rolling recommendation to someone else for an amp like an 8B w/o also stating that circuit operation will be changed by the sub. This is more than just sampling the different flavorings of tube; it's technically a circuit change too.
Steve
If you look on the HongKong threds,you will find many people running Kt66s in the 8B..Before you criticize it,why don't try and see for yourself.
I run kt77s and 6ca7s in mine as well as the KT66..Whats wrong with rolling tubes? People obviously like the effect.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
...do you know why the 8B cathode/g3 circuit is the way it is? I know I don't know for sure. I believe it's irresponsible to recommend what amounts to circuit changes to the amp w/o knowing this and the potential consequences of the changes. Just swapping out tubes and hoping for the best is insufficient IMO, esp when it's someone else's amp.
Understand that I'm not criticizing the sound of the rolled 8B or the tastes of those who do it. Rather, I'm criticizing those who unwittingly recommend making changes to an amp's circuit with no understanding of what they're changing (or even that they're making a change in the first place) and the possible consequences of those changes.
cathode is tied to the beam forming plate thru the 3.3 ohm resistor.Most of the 8B character comes thru its FB and transformer design.Yes,the some tubes tie 1 and 8 together but that isn't an issue that causes real problems that I have found.I will sweep my model 8 with kt77s again and kt66s and I will see if there is any issues with the FR being all over the place.In the meantime,you have an 8B,why not try some different tubes and see how you like it? Its not going to hurt the amp.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
Edits: 06/25/12
In the 8B it's 6.8Ω paralleled by 0.02µF. I doubt that a simple frequency sweep will reveal much of anything since I suspect the network has to do with either stability and/or response linearity into reactive loads. The 8B was advertised as unconditionally stable into any load with any signal and each output transformer was individually tweaked at the factory with a couple of trimmer caps in the feedback circuit to help assure this.
As far as trying different output tubes, I already stated that I tried NOS GEC KT77s and didn't care for them. I thought the Mullard/Amperex EL34s were best with the RFTs not far behind. RFTs are in there now. Based on this, I'm sticking with EL34 types, thank-you. YMMV.
The RFTs are my favorite of the EL34s because they have upper and lower detail that I felt was lacking in most EL34s including my Amperex D getters..I do like the metal base after they are on a while but I still get that mushy sound that kind of smears everything. The mids are nice tho..Try the kt66s grey bottles..Those are the ones I really liked in my 8 as well as the RI KT66 Gen..I may get the nines out and play around with them..They were given to me by one my of old audio mentors who bought them new in 1964 along with a Scott 4310 and s 7c..I never took the 7C or the 4310 and I ended buying a 4310 for 1600 a couple years ago.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
Edits: 06/25/12 06/25/12
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