Well, guitar amp is exactly you don't want to follow here!!! They bias so cold so you can drive it up to get the peak power. Believe me, I have a JCM900 Dual Reverb, I use two 6550 tubes and drive about 35W to 40W 50 to 80 Hz to break in the new speaker. I use DVM to measure the RMS voltage to set the output. I blew 2 tubes in 2 hours. I ended up had to use a variac with a 110 to 18V transformer to finish the breaking in.
As an owner of two Marshall 100W including one vintage Plexi 100W triple stack, Marshall is the utmost unreliable amp. I have so many blow out, I got my Plexi brand new in 73 and blown the rectifier twice the 3 years I owned it. The JCM900 is nothing but problem. I ended up gutted the inside and totally designed my own in. I even took out the PT and OPT and put in the new ones. You will have much better luck with the old BF and SF Fenders, they build like a tank.
Guitar amps are designed to have very high power if you don't have a continuous signal. You strum the guitar or playing sole, those are like spikes of high voltage. The average power is quite low. I did measurement by playing the guitar. Amps last even if you crank it up full and play loud, but you'll burn the amp with continue steady RMS voltage input.
As an owner of two Marshall 100W including one vintage Plexi 100W triple stack, Marshall is the utmost unreliable amp. I have so many blow out, I got my Plexi brand new in 73 and blown the rectifier twice the 3 years I owned it. The JCM900 is nothing but problem. I ended up gutted the inside and totally designed my own in. I even took out the PT and OPT and put in the new ones. You will have much better luck with the old BF and SF Fenders, they build like a tank.
Guitar amps are designed to have very high power if you don't have a continuous signal. You strum the guitar or playing sole, those are like spikes of high voltage. The average power is quite low. I did measurement by playing the guitar. Amps last even if you crank it up full and play loud, but you'll burn the amp with continue steady RMS voltage input.
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I've built a 1625 (12V filament version of the 807) pentode design - a very nice amplifier.
One of my favorite builds was a fixed bias single-ended pentode amplifier that allowed multiple output tube types to be rolled in. The EL34/KT88 was my - subjectively speaking - favorite tube to use. The 6L6GC "family" was noticeably harsher - whether it was a pair of Sovtek 5881s or Reflektor 6P3S-E, Japanese blackplate 6L6GCs, or some Valve Art 350Bs. Perhaps a better operating point was needed for the plate/screens and some feedback optimization. Never experimented that much since I was happy enough with Philips EL34s.
One of my favorite builds was a fixed bias single-ended pentode amplifier that allowed multiple output tube types to be rolled in. The EL34/KT88 was my - subjectively speaking - favorite tube to use. The 6L6GC "family" was noticeably harsher - whether it was a pair of Sovtek 5881s or Reflektor 6P3S-E, Japanese blackplate 6L6GCs, or some Valve Art 350Bs. Perhaps a better operating point was needed for the plate/screens and some feedback optimization. Never experimented that much since I was happy enough with Philips EL34s.
I too would like to see some simple 6L6 amp designs (5-20W?), since i have a couple of used 7581a (Phillips) that ive kept as spares, after replacing the quad in my DM120r rack head.
A cute simple and low thd design would be right up my alley (hifi amp). Id love to see some more, though admittedly i haven't researched much yet. With the increased headroom of the 7581a over the stock 6L6GC/GWB, id be interested to see if this would translate into reasonable power class A, pp or set, for a small kitchen stereo.
A cute simple and low thd design would be right up my alley (hifi amp). Id love to see some more, though admittedly i haven't researched much yet. With the increased headroom of the 7581a over the stock 6L6GC/GWB, id be interested to see if this would translate into reasonable power class A, pp or set, for a small kitchen stereo.
My first DIY amp was half a 6cg7 driving a 6l6gc into a Hammond 125E transformer (this was before the ESE series came out.) In many ways it bested the 2A3 amp I built next, even with the relatively low-fi OPTs. I don' t recall the exact operating points but b+ was in the high 300v range and I probably ran it close to max. dissipation. I need to revisit this some day.
So any comments on using PP OPT with lower primary resistance to lower 3rd harmonics by sacrificing output power? Something like 3K primary to 4 ohm speaker. Then use lower plate voltage.
The nice thing about 6L6 and EL34 is if you use grid bias, you can experiment with both as long as you connect pin 1 to the cathode and build in the provision to adjust the grid bias.
I use both 6L6 and EL34 in my designs in guitar amps, so these two are my logical choice.
The nice thing about 6L6 and EL34 is if you use grid bias, you can experiment with both as long as you connect pin 1 to the cathode and build in the provision to adjust the grid bias.
I use both 6L6 and EL34 in my designs in guitar amps, so these two are my logical choice.
Actually, the "6L6" is used a lot more often than it would seem. 807, 1625, 6BG6GA -- all of these are the same as the 6L6, just in different packaging. The 807 and 1625 are RF PAs that use glass envelopes and the then standard 5-pin and 7-pin sockets, and plate top cap connections to get higher voltages for more power, and to make it easier to construct an RF final as the input and output tuners can be shielded by the chassis itself. The 6BG6GA is a TV HD deflection type, either an 807 with an octal base, or a 6L6 with a top cap. I did a project with 807 finals, but it could have used any of the 6L6-oids.
As for harmonics, the 6L6 does feature some of the lowest THD specs. The big problem is that it also likes to make higher order harmonics. The Twin-T test shows a residual that looks like a sawtooth at 3X the frequency. That sounds horrible when running open loop. Local NFB (feedback 10% of the AC at the plate(s)) and gNFB will clear up that nastiness.
Maybe that's why you don't see more designs for this? It's easier to implement UL when you have a type that can more easily run the screen and plate at the same voltage. Maybe they don't have a handle on how parallel lNFB works? I've gotten excellent results from 807s despite what you sometimes see around the 'nets.
I notice when I open the GNFB in my guitar amp with 6L6, it got harsh sounding on top of slightly louder( very low loop gain). Maybe that what you are referring to. I don't particular like it even in guitar amp. But it seems like GNFB together with low loop gain tamed the harshness.
Maybe that's the reason Fender SF in the 70s put a 2000p on the plate of the 6L6. Must be on top of creating a dominant pole, it takes a little edge off.
Hi
I read through your article. That confirms a lot of my believes that DC coupling to a low impedance driver for the power tube is paramount. Also Cathode bias will cause blocking just like the effect of coupling cap charging up of the power tube, you go into deeper Class AB as the grid to cathode voltage becomes more negative after the passage of high amplitude signal.
I know you explained about the dB in gNFB in another post, I forgot and I forgot what thread was it on. Can you kindly write it out again? Thanks
Is there any chance you are willing to share your schematic. I understand if you don't want to.
Happy New Year
Alan
Decades ago I told my repair customers, "You know how Marshalls sound like they're going to explode any minute? That's because they are."
I, too, see little but EL34s in midpower hi-fi designs, and I, too, thought that the 6L6 was condemned for some reason....
the 6L6 was the answer to the clamor for more power, easy to drive and efficient designs...
these are the reasons they caught on with the masses....
these are the reasons they caught on with the masses....
dbv= 20log(Avcl/Avol)= 20log(1 + BAvol)
log: logarithm to the base of 10
B: Voltage gain of the feedback network
Attached
Thanks. I just want to double check
dbv= 20log(Avcl/Avol), not dbv= 20log(Avol/Avcl). Because dbv= 20log(Avcl/Avol) is negative all the time.
Thanks for your schematic, I need to read it.
Happy New Year.
Alan
😀
The art of guitar amp design could be summed up as making it sound like it's about to explode even though it isn't.
Problem is the Marshall kind of explode!!! The old Plexis had design problems. Using 1N4007 as rectifier diodes is not enough. Even as bridge rectifiers, the HT is close to 500V. The worst is they put the rectifier before the standby switch and reservoir cap, every time you turn on the standby switch, you have a huge current surge. I blew the rectifier twice in 3 years.
Then the amps do not short out the output if you forget to plug in the speaker cable. They are known to blow OPT.
Then the amps do not short out the output if you forget to plug in the speaker cable. They are known to blow OPT.
In fact I use 5gu4b in place of 5ar4 to drop peak voltage because of High mains here. Fortunately the 3a filiment vs 2a for 5ar4 is avail from PT. Always a tradeoff!
Or try KT66 (near 6L6 equivalent) for the cute bowl bottom
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Thankfully my plexy never exploded 😛
I hardly use it anyways 'cause at 520V into 4 EL34s it's just too darn loud. 100W? Yeah right, more like 120-175 depending on what valves you put in. 😎
Back to 6L6 for audio. In my living room I play with pp trioded 6L6GC. Still have to measure its performance and tweak things, but with powerdrive it has plenty of power. When I am alone I don't realise how loud I play, only when somebody enters and tries to talk to me. I'd say it's not too sharp sounding 😛
(My Bluesbreaker / Bassman has EF80 pentode mode outputs for ½W output! I live in an apartment... =D)
Considering triode mode, looking at the curves: http://www.audiomatica.com/tubes/img/6l6trio.gif
They seem to bunch up. Even with a CCS tail in balanced there will be quite a lot of 3H.
Compare to say 2A3 or 4P1L or 47 in triode mode.
Depends on the circuit, of course. Are you doing pentode or UL or triode?
Considering triode mode, looking at the curves: http://www.audiomatica.com/tubes/img/6l6trio.gif
They seem to bunch up. Even with a CCS tail in balanced there will be quite a lot of 3H.
Compare to say 2A3 or 4P1L or 47 in triode mode.
Depends on the circuit, of course. Are you doing pentode or UL or triode?
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