s-petersen
Scott
It will certainly benefit from the original OPT. Awesome on getting the 6L6's working.Do you have a scope and sig gen?
I Beg to DIFFER ! Westinghouse designed the 7591 in 1959 with sales starting 1960. It was designed for audio, NOT Industrial applications, but with most tubes you can find different applications. Sweep tubes as audio output tubes. etc. But the 7591 was designed to fit in a smaller envelope than the 6l6's of the time, while maintaining a 19W Maximum dissipation. These ARE NOT Korean War (1950-1953 note the dates Gary) Surplus tubes. And there is more than 1 new stock replacement. EH7591a in the bog bottle, Tung-Sol 7591A re-Issues (original size envelope), and the ever popular on the guys with more $$$ than (add your own expletive or commnet here) JJ's with SIDS. Granted they are all made in Russia, but the QC on the Tung-Sols and the EH's have much improved over the last 5-6 years. JJ's are still a flip of the coin as to whether it will encounter SIDS within the 1st 2 weeks.They are obsolete surplus korean war tubes with industrial numbers with one choice new.
Yes on scope and signal generator.It will certainly benefit from the original OPT. Awesome on getting the 6L6's working.Do you have a scope and sig gen?
You may be right on the tubes being for audio , I was guessing from the industrial numbers on them that they were most likely military surplus tubes or some other type of applications. The russian and china tubes are better now then they were at first but other than the Groove Tube 6L6 China GE and Svetlana EL34 they still don't sound very good to me. With a mix of NOS and them you can get a pretty good sound sometimes.I Beg to DIFFER ! Westinghouse designed the 7591 in 1959 with sales starting 1960. It was designed for audio, NOT Industrial applications, but with most tubes you can find different applications. Sweep tubes as audio output tubes. etc. But the 7591 was designed to fit in a smaller envelope than the 6l6's of the time, while maintaining a 19W Maximum dissipation. These ARE NOT Korean War (1950-1953 note the dates Gary) Surplus tubes. And there is more than 1 new stock replacement. EH7591a in the bog bottle, Tung-Sol 7591A re-Issues (original size envelope), and the ever popular on the guys with more $$$ than (add your own expletive or commnet here) JJ's with SIDS. Granted they are all made in Russia, but the QC on the Tung-Sols and the EH's have much improved over the last 5-6 years. JJ's are still a flip of the coin as to whether it will encounter SIDS within the 1st 2 weeks.
Thanks for the info about the console and year ! As far as the tubes go it is Just my choice, I like The Sound of The RCA Holy Grail Blackplate 6L6 tubes and There are many other choices with NOS 6L6 tubes. The voltages were in the right place for the RCA's also ! . I do not think that the origional 7591 tubes sound better than RCA blackplate 6L6 tubes !.This set up is out of a '62 Ambassador Console. The Amp is actually a 660A. Why go to a 6L6GC when it's optimized for 7591's?
Thanks for the info about the console and year ! As far as the tubes go it is Just my choice, I like The Sound of The RCA Holy Grail Blackplate 6L6 tubes and There are many other choices with NOS 6L6 tubes. The voltages were in the right place for the RCA's also ! . I do not think that the origional 7591 tubes sound any better than RCA blackplate 6L6 tubes !.
Thanks for that detailed information for me to look into , my plan was to put another Fisher output transformer back on it . I disabled the NFB already and the amp has good clean output , amps are like building a race car, you can over do things sometimes ! A good or winning combination can happen sometimes when the right parts come together, the tone quality of The RCA 6L6 blackplate tubes is more important to me then the Fisher and 7591 specs are to me ! . Certainly voltages are important to be within a certain range but they are not the only thing to consider and they can be over or under spec by 20% with no problems usually, good parts and tighter tolerances help, I was thinking a smaller 3 watt grid resistor would be safer than the 10 watt I used. The caps and resistors are not exact values to begin with and neither are the replacements ! . I am not trying to squeeze every bit of power out of it or to make it the best Fisher 600A on the planet , I just want a reasonably nice tone out of it and to me it is pretty loud already , I do not agree with lowering the voltages, I upped the screen resistor already to 500 ohm from 470 ohm and it sounds very good there !. I am not one to lower voltages in my amps. Output Tube specs are being exceeded all the time with good results. There are two rating systems also and that must be considered when you are comparing tube types. That is valuable information you gave me and I thank you for the deeper thought ! , I used to get that kind of input from James Henderson of James Audio in FT. Worth before he passed on and I learned a lot from it. But he usually in the end told me to do it my way and added a bass cap or something minor to my designs. I would try his ideas out and if I liked the sound I would use it if not I would remove it. It always came back to the sound over the spec in the end !. I am not a full time tech this is just a hobby for me and I am self taught from a WW2 course, I admit that I still have a lot to learn but I have basic electronics training and I have always been pretty good at diagnostics. This amp has two balance pots and a bias adjustment , I do not like new tubes in any of my amps. Thanks for the time Dave !Your project is very doable, but doing it correctly depends on how much re-engineering capability you have -- not only in the understanding of theory, but test equipment capability as well. I say the word "correctly" based on either making the amplifier simply "work" (i.e., operate reasonably well) dependably and safely with the alternate tubes, or having it produce all the performance of the original design that Fisher intended for it with the alternate tubes installed. That is two very different things. In either case, basic issues to address include:
1. Mismatched OPTs. The original Fisher OPT is no doubt significantly superior to the replacement piece. It would be best to get an original replacement transformer. Otherwise, understand that there will be performance given up by using the replacement unit installed.
2. 6L6 class tubes have about half the Gm of 7591 tubes, therefore, the 6L6 tubes will require about double the signal drive voltage and double the required grid bias voltage. But these amplifiers do not include a bias supply but rather, used the heater elements in the audio tubes on the adjoining tuner preamp to provide cathode bias to the output stage. This will clearly need to be adjusted to suit. The increased bias voltage required will represent a loss of B+ voltage, so power output will be reduced with such a move. NFB will be reduced as well due to the lower output stage gain, so that if it is restored to its original value, the input sensitivity of the unit will be cut in half -- again, all owing to the fact that the Gm of 6L6 tubes is much lower than that of the 7591.
3. To promote a proper impedance match to (at least) the original OPT, the screen voltage will need to be dropped to about 350 vdc. But doing this by simply upping the value of the screen grid dropping resistor will produce poor results as then the regulation of the screen grid voltage will be very poor.
All these things would need to be addressed to achieve the performance of the original design. If you just want to get it working reasonably well, I would consider installing a separate negative bias supply and an EFB™ screen grid regulator. This will allow the tubes to be biased properly and develop a similar amount of power output (if not more) than that of the original tubes. But the NFB, sensitivity, and stability issues would all have to be sorted out from there as desired. Is there any reason you don't want to use new manufactured tubes as used in the original design in the unit?
Either way, good luck with your project!
Dave
I do not own a square wave generator and I dId not plan on buying one either ! , The scope check will have to do ! Do You see anything wrong with this scope pattern at 15 watts output ?If it were mine, I would check it with 10k square waves, and see what it's doing.I have had amps that sounded "ok", but sounded much better after correcting the square waves.
Why would I want to check for square waves on a analog tube amplifier that has round waves ? Seems like the wave patterns and slopes would be more important ! The linear output matching the tube charts is what I always thought I was after ? The Fisher analog round wave tuner it came with is my input.I don't see sign waves, most basic checking is done at 10khz at 1 watt, it should look like a clean sign wave if that is your input.
Why would I want to check for square waves on a analog tube amplifier that has round waves ? Seems like the wave patterns and slopes would be more important ! The linear output matching the tube charts is what I always thought I was after ? The Fisher analog round wave tuner it came with is my input.
Do I really need all of that if the amp already sounds good to me ?Need a distortion analyzer, tone generator and load box for a start.
Thanks for explaining what the purpose was for , and it is not that important to me. I'm using the Fisher analog tuner and it is solid state rectified so I already understand there a few limitations built into the amps sound, that is why I decided on using the 6L6 tubes.I believe a square wave would represent a most difficult type of signal to reproduce accurately, so viewing square wave performance will help to see how well the amp is performing. Ringing instability and high frequency rolloff are easily visualized with a 10KHz square wave and oscilloscope. If your objective is the most accurate, uncolored reproduction of your source material, this might be important.
Do I really need all of that if the amp already sounds good to me ?
In the end all that matters is that it pleases you.Do I really need all of that if the amp already sounds good to me ?