Hello:
I am hoping to build the noted amp as designed by RobRobinette. He does 2 designs one with 12au7(1W) one with ef80(2W) as the output stage tubes. I am looking for a bit more than 1 Watt output and I do not have any ef80 tubes. Has anyone done a similar pentode pushpull design with other output tubes. I am looking for output in the 1,2,3W range. I have a Hammond 125b output transformer.
Thanks for the help and discussion.
I am hoping to build the noted amp as designed by RobRobinette. He does 2 designs one with 12au7(1W) one with ef80(2W) as the output stage tubes. I am looking for a bit more than 1 Watt output and I do not have any ef80 tubes. Has anyone done a similar pentode pushpull design with other output tubes. I am looking for output in the 1,2,3W range. I have a Hammond 125b output transformer.
Thanks for the help and discussion.
The EL84/6BQ5 - that would be my immediate choice. My (modded Class5) Marshall Plexi5 uses one. The Hammond 125BSE will be perfectly fine with it.
T
T
Hello @tubelectron:
Thanks for the quick reply. I was more looking for a pushpull pentode set up in the low wattage range. Single ended will have a different distortion profile than pentode. Rob mentions that on his site. That is why I was asking about PP designs.
How is the Celestion Greenback? Rob mentions it on his site. It sure is pretty.
Thanks for the quick reply. I was more looking for a pushpull pentode set up in the low wattage range. Single ended will have a different distortion profile than pentode. Rob mentions that on his site. That is why I was asking about PP designs.
How is the Celestion Greenback? Rob mentions it on his site. It sure is pretty.
@SCD : below there's a sample of my little VOX AC3 Top Boost - I think that it's enough convincing about the overdriven tone of a SE EL84/6BQ5. The guitar is Strat, and the sample is recorded with my Roland R-05 pocket digital recorder :
And here's the amp - a tube rebuilt from a mint-defunct 1968 VOX Escort mains-battery SS amp :
There's only an ECC83/12AX7, an EL84/6BQ5 and a 5" speaker :
Preamp pentodes (better) or triodes (worse) do not have "the sound" when used in output stages - that's at least the conclusion I draw after many attempts.
A power pentode or tetrode is way better sounding, I mean : more close to a good tube amp overdriven tone (assuming the preamp section has been well designed too) : ECL82, ECL86/6GW8, EL90/6BQ5, 6V6, EL95, etc... In pentode connection, not triode (less power yes, but same current drawing, less sensivity and muddy tone).
I don't think that a push-pull of EF80, EF85, EF184 would be able to compete with a SE EL84 power pentode in terms of overdriven tone. But it's me, OK ? 😉
After all, a test will tell you what is best for your taste ! 🙂
Sorry, I did not recorded samples of my Marshall Plexi5, but yes indeed, the Celestion Greenback 10 is a "Tone Safe Value" ! 😎
T
And here's the amp - a tube rebuilt from a mint-defunct 1968 VOX Escort mains-battery SS amp :
There's only an ECC83/12AX7, an EL84/6BQ5 and a 5" speaker :
Preamp pentodes (better) or triodes (worse) do not have "the sound" when used in output stages - that's at least the conclusion I draw after many attempts.
A power pentode or tetrode is way better sounding, I mean : more close to a good tube amp overdriven tone (assuming the preamp section has been well designed too) : ECL82, ECL86/6GW8, EL90/6BQ5, 6V6, EL95, etc... In pentode connection, not triode (less power yes, but same current drawing, less sensivity and muddy tone).
I don't think that a push-pull of EF80, EF85, EF184 would be able to compete with a SE EL84 power pentode in terms of overdriven tone. But it's me, OK ? 😉
After all, a test will tell you what is best for your taste ! 🙂
Sorry, I did not recorded samples of my Marshall Plexi5, but yes indeed, the Celestion Greenback 10 is a "Tone Safe Value" ! 😎
T
Power triodes sound bland, power pentodes (or tetrodes) have balls.
Personally agree with a single EL84 or 6V6 but if you Insist on push pull, a couple 6AQ5 can fit the bill.
Personally agree with a single EL84 or 6V6 but if you Insist on push pull, a couple 6AQ5 can fit the bill.
Scot,
Lots of options here.
You can look up the "$100 guitar amp challenge" and get some ideas for output tubes.
With the 125B you can use some pretty high Rp tubes, did you buy any small pentodes in the $1 tube sale days?
Another option is tubes like 6K6 and 6Y6, an old trick is to swap them into 6V6 amps for use as practice amps.
Lots of options here.
You can look up the "$100 guitar amp challenge" and get some ideas for output tubes.
With the 125B you can use some pretty high Rp tubes, did you buy any small pentodes in the $1 tube sale days?
Another option is tubes like 6K6 and 6Y6, an old trick is to swap them into 6V6 amps for use as practice amps.
I've never built such an amp, but as a guitar player and 'knower' of JCM800, I don't know how you're going to get 'that sound' without actually building/buying a JCM800. You could certainly construct the preamp side with requisite values/voltages... and mimic the power side with a p/p. So what's lowest level pentodes? 6au6?
wow: Thanks for all the comments and advice:
I am building this for a friend as a gift. He wants something small that has great sound. I did a search and landed on RobRobinette's site. When I look through my tube collection I do not have the EF80 tubes he built the output stage around for his 2W option. Today, I have been looking through my tube manuals and and am considering 6cl6, 6as6, 6cm6, 6aq5, 6ca5, 6bf5. All of these should give in the range of 1-3W and are pentode output tubes.
@leadbelly: I got ~ 500 7,9 pin tubes from a retire fellow in Victoria for way too cheap and have been scrounging tubes for a long time. I am a big fan of "not audio tubes". That comment is one I love to read, because it means somebody does not know how to design amps or understand tube manuals and is only looking for the mass market over priced stuff. There are many gems in the rest of the tube world if you know a minor amount about design and can navigate around a tube manual. Thanks for idea about the 6k6, 6y6, They look interesting I will look them up. And I will check out the Guitar amp challenge.
My quest continues!
I am building this for a friend as a gift. He wants something small that has great sound. I did a search and landed on RobRobinette's site. When I look through my tube collection I do not have the EF80 tubes he built the output stage around for his 2W option. Today, I have been looking through my tube manuals and and am considering 6cl6, 6as6, 6cm6, 6aq5, 6ca5, 6bf5. All of these should give in the range of 1-3W and are pentode output tubes.
@leadbelly: I got ~ 500 7,9 pin tubes from a retire fellow in Victoria for way too cheap and have been scrounging tubes for a long time. I am a big fan of "not audio tubes". That comment is one I love to read, because it means somebody does not know how to design amps or understand tube manuals and is only looking for the mass market over priced stuff. There are many gems in the rest of the tube world if you know a minor amount about design and can navigate around a tube manual. Thanks for idea about the 6k6, 6y6, They look interesting I will look them up. And I will check out the Guitar amp challenge.
My quest continues!
You could look at something like a 6KV8. A little (high gain!) pentode in the right power range, with a 65 mu triode in the bottle. A pair can make a gain stage, phase splitter and push pull pair good for up to 5 watts. And can easily be dialed down into the 1-3 watt range - works nicely on low B+. There is the similar 6KR8 with the same pentode and a very nonlinear variable-mu triode in there if a lot of 2HD floats your boat.
I'm also convinced that »the« JCM800 sound requires a pair of pentodes in PP. I'd opt for EF80's indeed, as they're like sweepings here in Europe. Other choices would be 6AU6, EL95 (both as yet suggested), 6DX8 (their high µ triodes would serve well as the PI), 6AB8. As the OPT in a geetaar amp, PA line transformers would suffice. A 100 V one with a 0.625 W terminal provides a primary impedance of 16 kΩ, with the 2.5 W terminal as the CT.
Best regards!
Best regards!
I have built several small push pull amps in sizes from 1 to 10 watts. The mentioned 6AU6 will get to a bit over a watt in push pull but doesn't really like doing it. They were not bad for a bluesy sound but wouldn't clip hard no matter how hard you drove them. I currently have only one working guitar amp, but a few more are in the planning, simulating and breadboarding stages. The results will be posted here, but it may take a while.
My daily use amp is a series heater string creation made for the Hundred Buck Amp Challenge (a sticky at the top of this forum). I rebuilt it several years after the contest ended, morphing it into an amp that I would actually use. It runs a pair of 32ET5 tubes in push pull and makes 4 watts cranked to 11 on 165 to 170 volts of B+ through a $5 Parts Express line matching transformer as the OPT. I got a bit more power with a real OPT, but I don't remember how much. The schematic is included. The input stage is a contraption I created called the saturator It is basically a variable gain amp with more gain than an SLO-100 at full on. The pot can turn it down to "single 12AX7" levels at about 1/3 up.
I built a tiny 4 tube amp that could be stuffed inside a cavity in a solid body guitar. Tubes were selected for low heater current power. I wound up with a little subminiature tube for the gain stage, which was later replaced with a 6AK5 7 pin pentode. It was loaded with a CCS made from an LND150 mosfet which fed a second LND150 that was the PI. That fed a pair of 6AK6 pentodes, chosen for their wimpy 0.95 watt heater power. The pair of 6AK6s put out almost three watts on a B+ voltage in the 200 to 220 volt range depending on the charge in the 7.2 volt RC car battery and the mood of my crude DIY boost converter. It sounded far better than the 6AU6s did for about half the total power consumption. It drove a 3 inch speaker from Parts Express that was rather inefficient, but loud enough to induce some infinite sustain moments on some notes. The amp in the guitar thing along with a bunch of other neat stuff dissapeared in the 1200 mile move out of Florida and year long storage of stuff in various locations while my current house was being built adventure. I built it because I used to play a solid state amp in the guitar thing as a kid at the Lafayette Radio Electronics store in Florida. I was fascinated with it at the time, but don't think that I'll ever do it again.
The 6AQ5 tube can bang out 10 watts per pair in push pull but can be run at a lower power. There are other 7 pin tubes that were intended for SE use in a radio or TV set. I have played with most of them and 3 to 5 watts from a pair on a relatively low voltage is possible. The 6CU5 is a 6 volt 50C5 which is a derivative of the 6W6. Those can put out almost 20 watts in push pull if fed 340 volts, and at least 5 watts on 165 volts. The 6CA5 is a lower power version of the same tube.
My daily use amp is a series heater string creation made for the Hundred Buck Amp Challenge (a sticky at the top of this forum). I rebuilt it several years after the contest ended, morphing it into an amp that I would actually use. It runs a pair of 32ET5 tubes in push pull and makes 4 watts cranked to 11 on 165 to 170 volts of B+ through a $5 Parts Express line matching transformer as the OPT. I got a bit more power with a real OPT, but I don't remember how much. The schematic is included. The input stage is a contraption I created called the saturator It is basically a variable gain amp with more gain than an SLO-100 at full on. The pot can turn it down to "single 12AX7" levels at about 1/3 up.
I built a tiny 4 tube amp that could be stuffed inside a cavity in a solid body guitar. Tubes were selected for low heater current power. I wound up with a little subminiature tube for the gain stage, which was later replaced with a 6AK5 7 pin pentode. It was loaded with a CCS made from an LND150 mosfet which fed a second LND150 that was the PI. That fed a pair of 6AK6 pentodes, chosen for their wimpy 0.95 watt heater power. The pair of 6AK6s put out almost three watts on a B+ voltage in the 200 to 220 volt range depending on the charge in the 7.2 volt RC car battery and the mood of my crude DIY boost converter. It sounded far better than the 6AU6s did for about half the total power consumption. It drove a 3 inch speaker from Parts Express that was rather inefficient, but loud enough to induce some infinite sustain moments on some notes. The amp in the guitar thing along with a bunch of other neat stuff dissapeared in the 1200 mile move out of Florida and year long storage of stuff in various locations while my current house was being built adventure. I built it because I used to play a solid state amp in the guitar thing as a kid at the Lafayette Radio Electronics store in Florida. I was fascinated with it at the time, but don't think that I'll ever do it again.
The 6AQ5 tube can bang out 10 watts per pair in push pull but can be run at a lower power. There are other 7 pin tubes that were intended for SE use in a radio or TV set. I have played with most of them and 3 to 5 watts from a pair on a relatively low voltage is possible. The 6CU5 is a 6 volt 50C5 which is a derivative of the 6W6. Those can put out almost 20 watts in push pull if fed 340 volts, and at least 5 watts on 165 volts. The 6CA5 is a lower power version of the same tube.
I did not know anything about the EF80 until Google told me that it was a US 6BX6, which is not too common. It is however one of the many second generation TV IF amplifier tubes, and there are many variations of them.
The EF184/6EJ7, the 6JC6, 6JD6 and 6KT6 are all quite similar and share a compatible pinout. All of these should work in the Robinette JCM800 micro.
The EF184/6EJ7, the 6JC6, 6JD6 and 6KT6 are all quite similar and share a compatible pinout. All of these should work in the Robinette JCM800 micro.
The Hammond 125b is a push-pull output transformer.
These often won't work very well with single ended circuit.
You run the risk of saturating it with the DC bias, since it doesn't have an air gap.
For just a couple of Watts, I think 6AK6 tubes are pretty nice to use.
Enough for about 3-4W PP in Pentode mode or about 1.5W PP in Triode mode.
Alternative could be EL91.
Another advantage is the 150mA of heating current (200mA for the EL91)
These often won't work very well with single ended circuit.
You run the risk of saturating it with the DC bias, since it doesn't have an air gap.
For just a couple of Watts, I think 6AK6 tubes are pretty nice to use.
Enough for about 3-4W PP in Pentode mode or about 1.5W PP in Triode mode.
Alternative could be EL91.
Another advantage is the 150mA of heating current (200mA for the EL91)
EF184/6EJ7 in fact was EF80/6BX6's successor here in Europe for TV IF strip service. Due to it's rather high gm, this frame grid tube helped to save one IF stage (two EF184's instead of three EF80's). Three EF184's also were found in some color TV's to drive the CRT grids, with a PL802 to drive the cathodes.
Best regards!
Best regards!
good evening gentleman:
Thank you again for the comments and advice.
I have sent a note off to Rob Robinette to see what he thinks. I suspect he has explored many options with this project.
I am very thankful to get so many helpful ideas and comments.
Today I started to bend up the aluminium for the chassis. I had salvaged an old cube van truck box for some of the materials. My buddy had used the box for a grow room for a few years until mice and other vermin got in and messed with the crop. The roof finally caved in and he needed to make it go away. I helped and could not let all that lovely sheet aluminium go to the scrap yard. I have a few big chunks that I have been using for various projects. I love to repurpose stuff. Now a piece of it will be a guitar amp for another friend as a gift.
As they say keep the love flowing around the circle.
Thanks again.
More to follow
Thank you again for the comments and advice.
I have sent a note off to Rob Robinette to see what he thinks. I suspect he has explored many options with this project.
I am very thankful to get so many helpful ideas and comments.
Today I started to bend up the aluminium for the chassis. I had salvaged an old cube van truck box for some of the materials. My buddy had used the box for a grow room for a few years until mice and other vermin got in and messed with the crop. The roof finally caved in and he needed to make it go away. I helped and could not let all that lovely sheet aluminium go to the scrap yard. I have a few big chunks that I have been using for various projects. I love to repurpose stuff. Now a piece of it will be a guitar amp for another friend as a gift.
As they say keep the love flowing around the circle.
Thanks again.
More to follow
I built a 5E3 with 6AK6's. Had a recording of it but Microsoft deleted it. Through a good speaker you would not know it was a puny 7 pin bottle.
Yes, your »good speaker« is as well an important key for the desired sound. It's cause can be seen in the symbiosis of a PP pentode amplifier with modest NFB and it's humps and notches in the frequency response and impedance curves.
Best regards!
Best regards!
hello again folks:
I have decided to build a test mule for the output stage. The front end will be as designed by RobRobinette.
To accommodate for the various differing tube pin outs I will make a simple little box with pairs of 7, 8 and 9 pin sockets I will hook up 4mm probe inserts to the pins. I should able to play with most options this way. The test box will become a permanent part of my work bench for other future design explorations. It will also give me a chance to explore the sheet metal bender for making chassis boxes. I have some time to wait as usual for parts to arrive from various suppliers.
As this is for a guitar amplifier, when it comes to testing the output stage I could use some help with which parameters to focus on. Do I want a clean output stage or am I after some level of distortion etc. A little discussion with be very useful here.
The tools I have are: various DMM, Digital Signal generator, Siglent 1052dl Oscilloscope. Variac, 30vdc power supply.
Thanks again for all the help.
The road goes ever on.
I have decided to build a test mule for the output stage. The front end will be as designed by RobRobinette.
To accommodate for the various differing tube pin outs I will make a simple little box with pairs of 7, 8 and 9 pin sockets I will hook up 4mm probe inserts to the pins. I should able to play with most options this way. The test box will become a permanent part of my work bench for other future design explorations. It will also give me a chance to explore the sheet metal bender for making chassis boxes. I have some time to wait as usual for parts to arrive from various suppliers.
As this is for a guitar amplifier, when it comes to testing the output stage I could use some help with which parameters to focus on. Do I want a clean output stage or am I after some level of distortion etc. A little discussion with be very useful here.
The tools I have are: various DMM, Digital Signal generator, Siglent 1052dl Oscilloscope. Variac, 30vdc power supply.
Thanks again for all the help.
The road goes ever on.