I have an audio amp a friend brought me in hopes I could help him. The amp has 6B4 out put tubes that appear to be run in paralle. I have no idea what the preamp tubes are, I am hoping some one will recognize the circuit.
Here are some pics. TIA
BL
Here are some pics. TIA
BL
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Next. It may have missing wires from the bottom tube as there is no input to it.
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I need to say your point-to-point workmanship is very nice.
But yeah maybe ditch the Rifas, especially if they are vintage. They are famous for the wrong reasons...
But yeah maybe ditch the Rifas, especially if they are vintage. They are famous for the wrong reasons...
It's pushpull. This looks like cathode bias output stage and it also looks like in addition to 6B4G you can also run EL34/6CA7 UL connected in it. Long ago I heard a very, very similar amplifier with 6B4G driving a pair of Quad ESL speakers. I believe that one was equipped with 6DJ8 or 7308. Sounded very good. Charlie King built one or more of these in the late 1990s.
Be careful not to put a lot of stress on the leads of those Holco resistors, I had a lot of problems with ones that went noisy, they are otherwise fine resistors. (High voltages can cause noise problems too - if that is a problem replacing the 47K plate resistors should fix it. The RIFA caps probably ought to be replaced.
Be careful not to put a lot of stress on the leads of those Holco resistors, I had a lot of problems with ones that went noisy, they are otherwise fine resistors. (High voltages can cause noise problems too - if that is a problem replacing the 47K plate resistors should fix it. The RIFA caps probably ought to be replaced.
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This may be one of those those builds, this amp was supposed to have been built by Mr King. We looked all over to find a schematic to figure out exactly what it is, but alas found nothing. So I decided to draw the circuit but lacked the knowledge to figure out the cascode split phase inverter. That’s when I turned to this fine group of DIY’ers for help.
I am unsure if this amplifier worked when he bought it, but it end up with me not working. Now I believe I can recreate the schematic and get down to trouble shooting it.
Kevin I would be very interested in knowing more about that amp you mention or if anyone else has information.
BL
I am unsure if this amplifier worked when he bought it, but it end up with me not working. Now I believe I can recreate the schematic and get down to trouble shooting it.
Kevin I would be very interested in knowing more about that amp you mention or if anyone else has information.
BL
Well I'll be!. What goes around,, Wondering what happened to that amp.
Yeah, that was my last foray into tubes - 25 or so years ago. Sounded good enough that I ended up building six of them, five for my Connecticut Audio Society friends and one going out West somewhere. This one must be it. Recently bought one of the local units back for my bedroom system so I'd have tubes somewhere in the house. Ones locally are all still going strong - with original output tubes even!!
Yeah, the mystery tube is a 6DJ8/6922. Circuit is the Hedge differential cascode originally described in Wireless World in the mid 50's. Showed up in a few Stereo 70 front end board re-do's. One of Harvey Rosenberg's companies, Tube God maybe, made a PC board for it. Idea started when I heard directly heated triodes for the first time. Was enthralled! Acquired a 2A3 single-ended, DIY, stereo amp and loved it. Wanted a bit more power and decided, as I had a few Stereo 70 chasses lying around, to try and build up a PP version using as much of the 70 casework and iron as possible. Settled on 6B4's and got the output stage working, experimenting with ways to lower the B+ to not blow up the tube. Ended up with a choke input filter and actually used the small cap at it's input to "tune" the B+ to the voltage shown.
Going to continue this post separately, as with my luck I'll hit a wrong button and erase everything, Bet you've never done that. Did spent a long time evaluating; building; listening to various input/driver circuits - more later
Yeah, that was my last foray into tubes - 25 or so years ago. Sounded good enough that I ended up building six of them, five for my Connecticut Audio Society friends and one going out West somewhere. This one must be it. Recently bought one of the local units back for my bedroom system so I'd have tubes somewhere in the house. Ones locally are all still going strong - with original output tubes even!!
Yeah, the mystery tube is a 6DJ8/6922. Circuit is the Hedge differential cascode originally described in Wireless World in the mid 50's. Showed up in a few Stereo 70 front end board re-do's. One of Harvey Rosenberg's companies, Tube God maybe, made a PC board for it. Idea started when I heard directly heated triodes for the first time. Was enthralled! Acquired a 2A3 single-ended, DIY, stereo amp and loved it. Wanted a bit more power and decided, as I had a few Stereo 70 chasses lying around, to try and build up a PP version using as much of the 70 casework and iron as possible. Settled on 6B4's and got the output stage working, experimenting with ways to lower the B+ to not blow up the tube. Ended up with a choke input filter and actually used the small cap at it's input to "tune" the B+ to the voltage shown.
Going to continue this post separately, as with my luck I'll hit a wrong button and erase everything, Bet you've never done that. Did spent a long time evaluating; building; listening to various input/driver circuits - more later
Attachments
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Input/Driver Stage
Originally wanted to go “retro” (back in 2000!) and planned to use octals, specifically 6SN7’s. Morgan Jones’s great tube audio design book did some rolling of them and I acquired a small stable of his fabs. To find a playpen, I found a (physically) small, broken, Bogen PA amp (using octals), stripped out everything but the power supply and proceed to build up various input/driver circuits. Then connected this “mule” (automotive test chassis slang) to the 6B4/70 chassis via an umbilical cable. To drive my Quad ESL63’s. Go ahead and laugh but this was to be the reason for the entire project. Know you WON’T laugh if you’ve experienced the 63’s. God they’re great
Anyway, after a number of listening sessions with a compatible friend (similar music and sound tastes) the design evolved to the Hedge circuit. Great detail. HOWEVER, ran into a separate problem At the time I was using a passive “preamp”, - Dave Slagle autoformer volume control and the N7’s just didn’t have enough gain. Fortunately was using John Broski’s (sp?) Tube Cad program, and found that once you had the circuit configured you could scroll thru a menu of tubes and see immediately how the input/output characteristics changed. DJ8 had maybe 6dB more gain so I tried those and bingo - just fine.
Notes on parts: Holco resistors - were highly thought of back then and haven’t had any known problems; Thanks Kevin for the caution about the leads. Output coupling caps: Certainly realize how these can affect the sound. Actually used 2 or 3 different types during the builds. These Rifa’s sounded quite good (or didn’t sound - as your case may be). Have had some other sizes/voltages explode and wouldn’t use them again. To play cap games I actually put in “above board” binding posts on one amp so I could quickly switch caps. Held a CAS meeting where we listened/evaluated a few different types of “high end” caps (circa early 2000’s). Very interesting and the subject of a discussion in itself.
Output tubes: Remember this was in 2000: was able to get NOS tubes fairly easily AND inexpensively; got and tried a number of Sylvania’s. Heart stopped a few months back when I looked at TODAY’S PRICE for them. Sovtek was also making nice ‘B4’s and used them in a few amps. Here’s the catch…found that unmatched tubes will result in 60/120hz hum thru the speakers. With well matched tubes, you can put your ear right up to the speaker and hear no hum. Did find that with a batch of tubes it was easy/quick to swap them and find a pair that resulted in negligible hum.
Aftermath: Upon extended listening, and depending on program content, the amp has prodigious bass BUT the lower end IS false/bloated. Tried adding a small amount of feedback. Found that 2-4 dB really tightened it up with no apparent side effects. HOWEVER try as I might, I could never get the hum resulting from my implementation(s) down to inaudibility, so I left it out.
When I moved from CT to NY my friend made me an offer I couldn't refuse and so I left behind amp, Quads and Slagle. They are still cranking today!
O yeah, the owner of the amp I bought back added an inrush current limiter (maybe a CL27 will have to check) in series with the power switch. Examined it's effect and realized that it really does supress the audible "clunk" the amp makes when you power it on. Which is good - gotta keep this puppy running till I finally enter/exit?? the drain.
BL - BEST WISHES in getting the amp working again - will help as I can - and let us know the results - only if positive tho
Originally wanted to go “retro” (back in 2000!) and planned to use octals, specifically 6SN7’s. Morgan Jones’s great tube audio design book did some rolling of them and I acquired a small stable of his fabs. To find a playpen, I found a (physically) small, broken, Bogen PA amp (using octals), stripped out everything but the power supply and proceed to build up various input/driver circuits. Then connected this “mule” (automotive test chassis slang) to the 6B4/70 chassis via an umbilical cable. To drive my Quad ESL63’s. Go ahead and laugh but this was to be the reason for the entire project. Know you WON’T laugh if you’ve experienced the 63’s. God they’re great
Anyway, after a number of listening sessions with a compatible friend (similar music and sound tastes) the design evolved to the Hedge circuit. Great detail. HOWEVER, ran into a separate problem At the time I was using a passive “preamp”, - Dave Slagle autoformer volume control and the N7’s just didn’t have enough gain. Fortunately was using John Broski’s (sp?) Tube Cad program, and found that once you had the circuit configured you could scroll thru a menu of tubes and see immediately how the input/output characteristics changed. DJ8 had maybe 6dB more gain so I tried those and bingo - just fine.
Notes on parts: Holco resistors - were highly thought of back then and haven’t had any known problems; Thanks Kevin for the caution about the leads. Output coupling caps: Certainly realize how these can affect the sound. Actually used 2 or 3 different types during the builds. These Rifa’s sounded quite good (or didn’t sound - as your case may be). Have had some other sizes/voltages explode and wouldn’t use them again. To play cap games I actually put in “above board” binding posts on one amp so I could quickly switch caps. Held a CAS meeting where we listened/evaluated a few different types of “high end” caps (circa early 2000’s). Very interesting and the subject of a discussion in itself.
Output tubes: Remember this was in 2000: was able to get NOS tubes fairly easily AND inexpensively; got and tried a number of Sylvania’s. Heart stopped a few months back when I looked at TODAY’S PRICE for them. Sovtek was also making nice ‘B4’s and used them in a few amps. Here’s the catch…found that unmatched tubes will result in 60/120hz hum thru the speakers. With well matched tubes, you can put your ear right up to the speaker and hear no hum. Did find that with a batch of tubes it was easy/quick to swap them and find a pair that resulted in negligible hum.
Aftermath: Upon extended listening, and depending on program content, the amp has prodigious bass BUT the lower end IS false/bloated. Tried adding a small amount of feedback. Found that 2-4 dB really tightened it up with no apparent side effects. HOWEVER try as I might, I could never get the hum resulting from my implementation(s) down to inaudibility, so I left it out.
When I moved from CT to NY my friend made me an offer I couldn't refuse and so I left behind amp, Quads and Slagle. They are still cranking today!
O yeah, the owner of the amp I bought back added an inrush current limiter (maybe a CL27 will have to check) in series with the power switch. Examined it's effect and realized that it really does supress the audible "clunk" the amp makes when you power it on. Which is good - gotta keep this puppy running till I finally enter/exit?? the drain.
BL - BEST WISHES in getting the amp working again - will help as I can - and let us know the results - only if positive tho
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All right!!!!!!!!
I was hoping to get some help with amp and it looks like we were able to close the circle back to the builder. I am a local tech who just piddles with these things and a friend brought this to me about a year ago and I gave up on it then. Here lately I ask him to bring it back and I’d make another run at it.
I drew the schematic out yesterday and my lord if Stellavox didn’t show up with his original schematic. Believe it or not we’re on vacation and I brought the chassis with me to study the chassis and to see if I could make any progress on it. Bingo!!!!!
Thx for all the help gentlemen.
BL
BTW Stellavox out west for now is Tulsa.
I was hoping to get some help with amp and it looks like we were able to close the circle back to the builder. I am a local tech who just piddles with these things and a friend brought this to me about a year ago and I gave up on it then. Here lately I ask him to bring it back and I’d make another run at it.
I drew the schematic out yesterday and my lord if Stellavox didn’t show up with his original schematic. Believe it or not we’re on vacation and I brought the chassis with me to study the chassis and to see if I could make any progress on it. Bingo!!!!!
Thx for all the help gentlemen.
BL
BTW Stellavox out west for now is Tulsa.