Help - PA Amp for guitar - increasing output?

[Shark_Sandwich]

Hi - first post here, and yes, I'm a newbie to tube amp design (trying to learn, though).

I have an old (1946) PA amplifier that I've converted for guitar use. Basically, I've changed the original microphone input to a more standard "Fenderish" input and I've replaced the caps. It's a 6L6 Push-Pull class A design, with a 70V output transformer and then a step-down transformer for the speaker. The input tubes are octals (6J7 then 6N7). I don't think I'm getting full power out of the output stage. I'm considering changing the two output transformers to a Hammond 1620 (6k6 primary impedance, 20W). Should this (potentially) increase my output? I'm assuming that the old transformers are not as efficient as a new one.

I'm also wondering if the input stage is not driving the output tubes hard enough. As I'm a newbie, I don't know how to figure this out, exactly. Any advice would be appreciated.

I've attached the schematic showing my modifications (grayed-out) in case that might help.

Thanks,

Don

[Satamax]

Ya could try dis! http://music-electronics-forum.com/showthread.php?t=354

[stokes]

In the schematic you attached the PI is a transformer coupled circuit.It has less gain than a typical modern tube PI.I would scrap it for a tube.I would also change that 70v transformer for one that couples your 6l6's directly to a 4,8 or 16 ohm speaker.

[Shark_Sandwich]

In the schematic you attached the PI is a transformer coupled circuit.It has less gain than a typical modern tube PI.I would scrap it for a tube.I would also change that 70v transformer for one that couples your 6l6's directly to a 4,8 or 16 ohm speaker.

Yes, I was wondering about that as well. I'm not going to add a tube PI (no room for it, really), and I am planning on switching to a single output transformer. Would updating the PI transformer help as well?

[Shark_Sandwich]

Ya could try dis! http://music-electronics-forum.com/showthread.php?t=354

I'm too much of a novice to understand the relationship between that link and my circuit. Any hints as to what I should look for?

Thanks.

[Enzo]

I would be assuming the amp does not have as much overall gain as you'd like. That is different from not having enough power. And this is not a guitar amp, so the gain structure of a guitar amp is absent.

And long before I would replace transformers, I think replacing caps would be a better step. Leaky interstage caps will weakien the signal, and leaky or dried out cathode bypass caps will sick the life out of it too. I see three 10uf bypass caps that ought to be replaced.

[stokes]

He said in the first post he did change the caps,but like Enzo says that aint a guitar amp,you wont get the gain from it as is,there is no point in putting a new interstage transformer,it is just a phase inverter with no gain at all,I would rewire the whole thing using 2 6SN7 type tubes since you have the 8 pin sockets,you could have 2 stages of gain before the signal hits the PI which is another stage of gain to the power tubes.What you have now is just 2 low gain stages hitting that trans/PI,I dont have any experiences with that PI,but I would think that tranny would load down the signal some,at best it has no gain whatsoever.

[Shark_Sandwich]

He said in the first post he did change the caps,but like Enzo says that aint a guitar amp,you wont get the gain from it as is,there is no point in putting a new interstage transformer,it is just a phase inverter with no gain at all,I would rewire the whole thing using 2 6SN7 type tubes since you have the 8 pin sockets,you could have 2 stages of gain before the signal hits the PI which is another stage of gain to the power tubes.What you have now is just 2 low gain stages hitting that trans/PI,I dont have any experiences with that PI,but I would think that tranny would load down the signal some,at best it has no gain whatsoever.

Yep, the caps have been changed. I'm not looking for more gain (overdrive), just more power. I can get a nice crunchy overdrive when I push the amp at full volume with humbuckers, and I use a pedal for more push if I need it. I'd just like more overall power. I do have an efficient speaker (Weber Blue Dog) in there.

I hadn't thought about switching out the tubes at all - the amp has a great clean sound already. Doesn't the 6SN7 have less amplification than the 6J7 (or 6SJ7) anyway? I think I'm really revealing my ignorance here.

[Satamax]

I'm too much of a novice to understand the relationship between that link and my circuit. Any hints as to what I should look for?

Thanks.

I was thinking "replace interstage trany by cascode phase inverter! " Gain, gain, gain

[stokes]

The 6J7 has an amplification factor of 20.The 6SN7 also has 20,but for the same tube space you get 2 stages,if you want more gain use the 6SL7 with a gain factor of 70,closer to the 12AX7.If you used these tubes with a 5E3 type PI you would have 4 stages of gain to drive those power tubes.With the current set up you have only 2 and the 6N7 has a gain factor of only 35,pushing a tranny,then the 6L6's.You just arent going to get any more power out of those 6L6's unless you push them harder.When I talk of gain I am not referring to overdrive necessarilly,but to get those power tubes to put out to their full power you need more preamp signal.

[Tom Phillips]

Don,

Your old amp may have only put out around 25 Watts when it was brand new. It probably started life with metal bottle 6L6s which are less capable than a the 6L6GC model that followed in later years. Even if you plunk in a new set of 6L6GCs you don’t automatically create a 50 Watt amp. The power supply and OT have to be up to the task too. Your power supply probably puts out 350V or less and who knows what the current capability of the transformer is. Bottom line is that you have to consider the weakest link. If we could see photos of the amp with a size reference if would help. Might be that your transformers are no bigger than those in a Deluxe Reverb amp. How much does the amp weigh?

Driving the tubes harder will not necessarily get you more power output. Just like yelling into a cell phone doesn’t improve the link. (Apparently most cell users believe otherwise)

BTW a transformer circuit can add gain. Depending on the turns ratio a transformer can add voltage gain or current gain. Just not both at the same time. Therefore, it can’t increase power between the primary and secondary.

There are some things you can do.
1) Make sure everything is working correctly. You are already doing that.
2) Install an OT that’s properly matched to the speaker load. (This is just because the amp currently has only that 70V output not because the 1940s transformers were inefficient or go bad over time)
3) Switch to more efficient speakers. Depending on what you have now that could make a big difference. (The equivalent of 2X, 4X or even 10X the power. Note it takes 10X the power to “sound” twice as loud)

OK. Enjoy your learning experience.

Regards,
Tom

[Shark_Sandwich]

Don,

Your old amp may have only put out around 25 Watts when it was brand new. It probably started life with metal bottle 6L6s which are less capable than a the 6L6GC model that followed in later years. Even if you plunk in a new set of 6L6GCs you don’t automatically create a 50 Watt amp. The power supply and OT have to be up to the task too. Your power supply probably puts out 350V or less and who knows what the current capability of the transformer is. Bottom line is that you have to consider the weakest link. If we could see photos of the amp with a size reference if would help. Might be that your transformers are no bigger than those in a Deluxe Reverb amp. How much does the amp weigh?

Driving the tubes harder will not necessarily get you more power output. Just like yelling into a cell phone doesn’t improve the link. (Apparently most cell users believe otherwise)

BTW a transformer circuit can add gain. Depending on the turns ratio a transformer can add voltage gain or current gain. Just not both at the same time. Therefore, it can’t increase power between the primary and secondary.

There are some things you can do.
1) Make sure everything is working correctly. You are already doing that.
2) Install an OT that’s properly matched to the speaker load. (This is just because the amp currently has only that 70V output not because the 1940s transformers were inefficient or go bad over time)
3) Switch to more efficient speakers. Depending on what you have now that could make a big difference. (The equivalent of 2X, 4X or even 10X the power. Note it takes 10X the power to “sound” twice as loud)

OK. Enjoy your learning experience.

Regards,
Tom

OK, here's my plan. I'm going to put in a new output transformer (the current design does have an appropriate 70V to 8ohm transformer as well), probably the Hammond 1620 (6k6 primary) and I'm going to change the 6n7 to a 6sl7 in order to increase gain. I do have an efficient speaker in there already, so that's taken care of.

I don't have a picture of the power transformer, but it's a monster. Very heavy & large. The transformer originally provided power for a light bulb used for sound decoding from 16mm film (via a photocell arrangement). I don't know the ratings on it, but I assume it's up to the task (never seems to get beyond warm). I believe that it puts out around 350V. I have switched to a 5AR4 rectifier to increase the B+ a bit, though. On the other hand, the output transformers (70V & 8ohm) are not the largest in the world.

By the way, it's not that this amp isn't loud already, I just want it to be louder (a bit) in order to keep up with a drummer at blues jams. I don't need high gain, just a bit more headroom.

I've learned a lot so far, but obviously I've much more to learn...

[Rob Mercure]

Hey SS,

Answered your other post first - should have checked this one out before posting there. For some reason no one has noticed that the 6N7 wasn't designed as a preamp tube but as a power output dual triode for Class B operation (battery power was precious in the 1930s so zero idle current counted for much) so I suspect that there is sufficient current available from the PI tranny for Class AB2 operation - why do you think the output sections is Class A? I suspect that you've got an AB2 circuit which would also provide more power output from the original metal 6L6 (or the 6L6Gs). Which opens some possiblilities for other circuit topologies.

While some have recommended that you pull the PI tranny if you like the amp's sound I'd reconsider that option. Interstage trannies, unless exquisitely, expensively built, were considered a "kiss of death" for HiFi - you need a large core with lotsa inductance for the lower end while the internal capacitance kills the high end. But we ain't talking about HiFi here - just the opposite. The lowest note of a standard tuned guitar is just above 80 Hz and high end response beyond 10K Hz actually annoys some guitarists. So your tranny might constitute and nice little "band pass filter" that suits your needs - I've never found any transformer coupled circuit that I didn't like for guitar amp use (but those little Gibson PI trannies from the 60s were often undersized and I have converted a few to different PI topologies at customer request - but these weren't AB2 but AB1 anyway.) Hmmm, how big is your interstage tranny? How much voltage is dropped across resistor 20? 'Nother "hmmm" - 'tis a strange design - not sure why the designers would parallel one 6N7 (which can put out 10 watts by itself easily) and then cap couple it to the PI tranny - must have been post WWII materials shortages that dictated the tubes used.

Ah well, hope I've helped muddy the water some <grin>. Someone else suggested coupling the amp with the most efficient speakers you can afford and I second that suggestion adding that you should restore the amp first and replace capacitor 23 while you're in there cuz that's what driver the PI tranny.

Rob

[Shark_Sandwich]

Hey SS,

Answered your other post first - should have checked this one out before posting there. For some reason no one has noticed that the 6N7 wasn't designed as a preamp tube but as a power output dual triode for Class B operation (battery power was precious in the 1930s so zero idle current counted for much) so I suspect that there is sufficient current available from the PI tranny for Class AB2 operation - why do you think the output sections is Class A? I suspect that you've got an AB2 circuit which would also provide more power output from the original metal 6L6 (or the 6L6Gs). Which opens some possiblilities for other circuit topologies.

While some have recommended that you pull the PI tranny if you like the amp's sound I'd reconsider that option. Interstage trannies, unless exquisitely, expensively built, were considered a "kiss of death" for HiFi - you need a large core with lotsa inductance for the lower end while the internal capacitance kills the high end. But we ain't talking about HiFi here - just the opposite. The lowest note of a standard tuned guitar is just above 80 Hz and high end response beyond 10K Hz actually annoys some guitarists. So your tranny might constitute and nice little "band pass filter" that suits your needs - I've never found any transformer coupled circuit that I didn't like for guitar amp use (but those little Gibson PI trannies from the 60s were often undersized and I have converted a few to different PI topologies at customer request - but these weren't AB2 but AB1 anyway.) Hmmm, how big is your interstage tranny? How much voltage is dropped across resistor 20? 'Nother "hmmm" - 'tis a strange design - not sure why the designers would parallel one 6N7 (which can put out 10 watts by itself easily) and then cap couple it to the PI tranny - must have been post WWII materials shortages that dictated the tubes used.

Ah well, hope I've helped muddy the water some <grin>. Someone else suggested coupling the amp with the most efficient speakers you can afford and I second that suggestion adding that you should restore the amp first and replace capacitor 23 while you're in there cuz that's what driver the PI tranny.

Rob

Hmmm, with my limited understanding, I assumed that with a grounded cathode design, the output stage was inherently class A. I'm starting to see that that might not be the case - it depends on B+ voltage & idle current, correct? I'll have to measure the voltage drop across resistor #20 - will that tell me more about what class? (I assume that that cathode resistor (as well as #26) were used to set the bias).

I don't have any desire to remove the interstage transformer - I'd like to keep the circuit as "stock" as possible. I don't doubt that it has a filtering effect, although there is almost too much high frequency content when driven hard. I don't have any delusions of turning this amp into a modern guitar amp - quite the opposite - I'd prefer a more "vintage" quality for this beast. Really, I'm just looking for a bit more headroom at this point (perhaps I'll tweak the frequency response a bit later).

I did put in an efficient speaker (Weber Blue Dog) which sounds great, by the way, and I've replaced all caps. I assume that cap #23 will have a large impact in terms of frequency response.

Yes, I don't doubt that parts specs were dictated by the war. This amp was built in 1944, and I don't know when the original design came to be. In the box with the schematic diagram was also a replacement tube list in case the owner couldn't find the original spec tubes, likely from war shortage. I noticed that all of the tubes were very common ones at the time.

Yes, the water is getting muddier (for me, at least), but that's how you learn, right?

Thanks,

Don

[Satamax]

Don, just a daft idea which i've just had, why not get teh two 6N7 in serries instead of parallel???