KT-88 vs. EL34

First of all, thank you for your replies. I will address all of your questions to date. 
1. Amp is wired as a pentode (or tetrode, depending on how you view beam power tubes) and not triode. Often thought of triode wiring and even single ended using one of the tubes but the power hit would be too great, maybe.
2. Not using AR4x. Those were my wife's college grad presents and have been out of sight for over 50 years (we just celebrated our 50th anniversary). Hooked them up for fun and they worked!
3. I am currently using Canton Karat M70s, woofers facing out. Not too efficient and 6 ohm average. Current amp is SS, a B&K AVR-307 (150w x 7) in two channel analog mode, used for stereo only. I like switching in the tube amp however; its intrinsic linearity is a pleasure to listen to. No edginess (but a little bright). Based on everyone's good advice I would really like to try the conversion to EL34/KT77s. a few resistors and Jond's 4 Gold Lion tubes and I'll be set to go.

I will try to track down the old A-C forums to learn more; Thanks for the suggestion bdp24. Andy8400.

bpoletti, you may be right. I had heard however that some, including possibly the above responders, have converted and found the EL34 to sound "better" on the same amplifier so the design wasn't a variable. On the other hand, the outlay of this type of tube amp generally is relatively straightforward. The one important variable that can truly influence the character of a tube amp IMHO is its output transformers because, as an inductor, evenness, peakiness, etc. are most influenced here. It's possible that the different loading that the EL34s place on the primary could account for the differences in sound. What do you think?

I googled EL34 vs 6L6 and was surprised at how much info there was on such a specific topic, including YouTube videos of guitarists comparing them. Of course the tests were in regard to heavy metal guitar amps and not critical listening but I think I preferred noromance's 6L6 recommendation. Should be fun to try since I have a nostalgic interest in the 6L6 anyway!

My question now is: how different, if any, are the bias voltage settings between the EL34 (which I think I know) and the 6L6 (which I don't know)? Are they truly drop-in replacements? Thanks, Andy

Thanks for your thoughts, sfall. As it turns out, I had already bi-amped these speakers in the past, more for control than for power. I had used different amplifiers and not all four from the B&K and for the following reason... To use 4 or any more than 2 channels from the B&K, you must be in the digital mode, that is to go through the amp's DSP. Although virtually indistinguishable from 2 channel, I wanted to keep my signal path pure analog which bypasses the DSP altogether.

Also, my listening room is small by most standards and the 150W per channel from the B&K's humongous power supply is an overkill as it is. In fact, to handle the myriad problems a small listening room provides, I use a 31 band equalizer to tailor the sound... particularly the bass bloat from the nearby room corners (and for minor adjustments to the rest of the audio spectrum).

You may be thinking that the Canton Karats are too much speaker for a small room and you'd be right, especially considering the smaller model speakers that are available. However, it's what I have to work with so it is what it is. Thanks again, Andy

noromance, yes to both. My question was (is) what is the difference in bias voltage between the EL34 and the 6L6? I found some tube specs through Google and they look close in an AB final with the 6L6 about 90% of the EL34. The problem is, I'm not sure. I would hate to break the connection, but I can always fall back on inserting a milliammeter in series with the cathode and set the grid bias by the current recommended in the 6L6 tube spec. Or... pick a bias voltage below the point where the anodes (plates) start to turn red! Probably not the best idea but it actually could work(!).

sfall, I agree with you conceptually and, as mentioned, I bi-amped my system for some time but for bass control, not for more power. I could use the digital mode with four amps since the difference in sound from pure analog is virtually undetectable. However, I listen to mostly classical music and the complex harmonics from large orchestral music are detectable (barely, but subtly noticeable, nevertheless) when processed through the DSP. As said, I have subsequently maintained control through a graphic equalizer in lieu of bi-amplification.

That's why, getting back to my original OP question, I want to improve and possibly substitute my tube amp and get back to the basics of music enjoyment. Thanks for everyone's reply! Andy8400.

Hi. A final word from the OP... I did end up modifying the bias circuits of the Jolida 502 to replace KT88s with EL34s (essentially making it a 302).

I LOVE the difference in sound, especially for classical music (what I listen to most) particularly how the tube amp handles complex harmonics (think of 30 violins, etc.). The only bad news is that it puts my poor B&K into question. I had assumed any subtle harshness was from my MM cartridge which has replaced my MCs (a topic for another time) but the edginess during difficult passages was significantly reduced so I'm keeping my MM cartridge but switching to the EL34 amp.

For smaller ensembles such as jazz, folk and even rock, the differences between the amps are virtually indiscernible (I installed a switch box and listened to instant, direct comparisons). Bottom line: The EL34 is a terrific tube. Worth the trouble. Thanks for everyone's input above. Andy8400