Bryrohr

As a matter of fact I do have a dual trace (rather slow at 15Mhz but not a problem for audio stuff) scope, also have two 8 ohm @ 250 watt Dale power resistors, signal generator for both sine and square, variac, Audio Control SA3050 RTA, various Fluke multi-meters...Etc. I agree with the low signal when feeding the input square waves as I wouldn't want to fry the trannies or pwr sply. Tubes are all NOS and barely broken in with maybe 50 hours on them. However, I did feel the 7 volt deviation of bias spec could matter as this would represent nearly 25% of -30 volts. I understand that if I were to run the bias voltage to -37 volts it would lower the plate current but what effect on sound quality and output capacity would result in this change is a mystery to me. My basic knowledge tells me that if the amp performs well, output tubes arent running at too high a plate current and the amp sounds great at the higher (-30vdc bias) plate current, go with that. My major concern is why Altec printed two schematics with two very different bias levels and what they thought was correct. Do you think they later changed the original bias spec of -30 vdc to -37 in an attempt to make the amp more bullet-proof and have longer tube life, or do you think they first chose -37 volts and then they felt the plate current just needed to be higher for better operation and sound quality so they revised it to -30 volts? Honestly, I haven’t even checked to see if the bias supply circuit is capable of producing -37 volts DC. LOL. If it won’t, I can rest assured this is just a typo on Altecs behalf. I'm thinking I should just measure the plate current at either setting and see what value fits the EL34's recommended plate current curves the best.

Fanatic, I do have a scope I could look at the change with. I assume doing this with a dummy load (I also have) at or near the maximum rated output level of 40 watts at say 1kHz would probably be a good place to start? I'm guessing the bias level shouldn't have an extreme difference on the output capacity since the B+ will stay the same roughly and the output devices/trannies are the same as well, but if the bias is set radically too low the tube would idle too hot and power output would decrease significantly as would tube life. Is this a good assumption? B.

Hello, I recently purchased a pair of Altec 1568A EL34 based mono tube amps. They work excellent and I plan on keeping them all original as they just have too much vintage appeal to change them. One thing I have noticed is there are two versions of factory released schematics running around the web that appear to be identical except for one has a -30 volt bias level and the other has a -37 volt bias level. Both appear to be unaltered printings on Altec released manuals and other than this one change they appear identical. The amps I have were set at very close to -30 volts bias and operate fine at this setting and sound absolutely astounding. I have have many other power amps that one would think would simply blow these out of the water and I really didn't expect much with these and was pleasantly surprised. The only thing I can see would happen by changing the setting to -37 volts would do would decrease the idle current through the EL34s and probably increase tube life, however, would lowering the plate current in this manor change the sound that much as well? Any comments would greatly be appreciated. Thanks

Now everyone is just standing around dressed in a wooden barrel wishing they could afford panties! Thanks for the welcome my friend.

Hello Captain, I too own a pair of Altec 1570b amps. I appreciate what you had to say about the technical aspects of them as I am always open for new ideas and rarely sell what I have aquired if its vintage gear. A closed mind and a "move on to something else as soon as you don't like what you hear" type personality is not at all my style. I still own gear I bought back in the seventies from my geometry teacher. I still have the Marantz model 7 he sold me in 1978 and somehow it still keeps finding its way back into my system since it sounds better than every other preamp I have tried since then, and I cannot count how many pre's that's been. Well, back to the Altecs. I bought a pair about 10 years ago with the hope they would drive my Martin Logan CLSIIz's to levels never before heard. Back then I had read an article in Vacuum Tube Valley describing what T.T. in Fla was doing to them and I made some of the mentioned modifications (all purely component changes as I wanted to keep them vintage looking) before even listening to them stock. I kept the tube B+ supplies basically stock as I felt going to diodes and the likes would be a step backward, but I did add some additional capacitance along with the 1KV stock can for the B+ as I felt that could not hurt. Along with that I also replaced the caps in the LV supply as it was noisy as hell. The main thing I did though was change the 6V6 drivers to EL34's since in his article Tom felt that was where the real major weakness was. I also replaced all the old carbon composition resistors with low tolerance metal oxide resistors since many of the original resistors had drifted as much as 50% or more in value over time. I also replaced the coupling caps with Hovlands. They work well, sound okay, but honestly, they suffered significantly from high end roll-off driving the large electrosats so I eventually moved on to something with better low Z drive capabilities. Lately though, I have been looking over at them, pondering what they would have sounded like stock. After all, I'm sure the engineers at Altec had nothing but performance and dependability in mind when they designed them. I'm not saying Tom doesn't know his sh!t, but he's one man working with what is there to work with. The Altec engineers were able to design everything as they wanted it to be, they were designers in the "golden age of hi-fi" faced with the challenge of building a product that not only would stand the test of harsh situations, but also be versatile enough to fit nearly any application, and sound like a professional product would be expected to sound like in doing all that as well. I guess, with all that being said, I would have to wonder what they sound like stock. A very close friend of mine bought a pair from T.T and they sounded quite nice driving his highly efficient dynamic speakers but eventually moved on as well. He never keeps an amp for much longer than a year or two anyway so I cannot say he was displeased with Toms re-incarnations, he just had to move on as usual. I hate to admit it, but mine are being used stacked up as an end table in my listening room. I said I never let anything go, and I still have them LOL. I even saved every part I removed from them originally. So, I was thinking of leaving the supplies as they now are, (basically stock with updated caps) keeping the Hovlands and updated resistors in place, but going back to the 6V6's and associated resistors that were changed to use EL34's as drivers. I somehow feel the 6V6's triode drivers might sound better than the EL34's. And, what would be your thoughts on replacing the 12AX7 phase splitter with a 6DJ8 since the amps seem to have plenty of gain for hi-fi use. I've never been a big fan of the 12AX7 and the 6DJ8 would provide a lower impedance for the input of the 6SN7. Whats your take on all this? And by the way, what exactly of any value were those other guys trying to add to your comments, and why were thier panties in such a wad? I thought you were being very respectable in mentioning the obvious EXTREME HAZARDS of working around HV, and those guys acted like you spit in their face or something. All I can say is they must have felt as if their manhood was being threatend in a way that their mothers used to do when telling them to "zip up that jacket or you'll catch a death" LOL