Lafayette PA-30 into guitar amp

[AmberB]

Hello, all!
I've been working on an old Lafayette PA amp to turn it into a guitar amp.  When the amp was given to me to work on, it was mostly original, with a very tired capacitor can.  The amp uses a pair of 6L6s for output, a 5U4 rectifier tube, and as built, had two 6SC7 twin triodes and a 6SJ7 pentode in the preamp.  Since the preamp tubes are octals, I decided that I would stay with octal tubes and rewire the preamp sockets for 6SL7s. 
There's a fair amount of Ampeg amps that use 6SL7 tubes, and the Lafayette amp already had the Ampeg type tone controls, so I've been reworking the amp with the Ampeg circuits in mind.  I attempted to use the paraphase type PI circuit in this amp and ran into a problem that I couldn't solve, so I decided to change the PI circuit to a cathodyne type.  I'm starting to put that together using 47K resistors for the plate and cathode of the phase splitter.  The part I'm trying to figure out is the plate and cathode resistors for the gain stage half of the tube.  I haven't found an example of a cathodyne PI using a 6SL7, but there are a number of them using a 12AX7.  I wanted to stay with the 6SL7 because it's the more common octal twin triode in guitar amps.

[sluckey]

Look at this. Schematic calls for 6SN7s but 6SL7s work very well too...

     http://sluckeyamps.com/rocky/rocky.pdf

[AmberB]

Thanks for the schematic! 
Right now, I'm using 6SN7s to test the amp, all my vintage 6SL7s are rather microphonic...

[AmberB]

It's interesting how Ampeg used so many unbypassed cathodes in their preamp.  I'm assuming that you get higher signal output from a stage with a capacitor bypassed cathode.

[shooter]

 


Ampeg was a civilized amp, they didn't through ice water in your face like other amps, they knew a real guitarist could get the point across just fine 

[DummyLoad]

It's interesting how Ampeg used so many unbypassed cathodes in their preamp.  I'm assuming that you get higher signal output from a stage with a capacitor bypassed cathode.

you get more gain with a cathode bypass condenser installed. more gain is less headroom - E.g., less gain, will allow a larger signal input level before the out clips, which in turn yields greater headroom.

ampeg designers wanted less gain and greater headroom. they weren't designing rock-n-roll amps. 


--pete

[bmccowan]

I have converted several old PAs for guitar use. Mostly Bogens and Stromberg Carlsons. It they have a 6SJ7 preamp I typically keep it. If they have a dual triode I keep that. Both sound good - just different. I'm on travels now, but when I get home I have a Grommes PA that I just started - I'll watch your posts and compare notes. Pic of the Grommes - lots of room!

[AmberB]

With the cathodyne PI circuit, there's basically 2 ways to connect the output of the gain stage to the grid of the phase splitter.  You can connect direct from the plate to grid, or you can use a cap between the plate of the gane stange and grid of the phase splitter.  There are a few wiring differences between the 2 circuits, mostly having to do with the arrangement of the resistors for the grid bias on the phase splitter. 
I chose to use the cap coupled arrangement for this amp.  I'll have to draw up a schematic when I get everything wired up and tested.
I'm going to end up having 3 triodes in the preamp, not counting the PI tube, sort of an Ampeg-ish hybrid...

[sluckey]

With the cathodyne PI circuit, there's basically 2 ways to connect the output of the gain stage to the grid of the phase splitter.  You can connect direct from the plate to grid, or you can use a cap between the plate of the gane stange and grid of the phase splitter. 

Actually there's a third way. AC coupled with fixed bias.

     http://www.valvewizard.co.uk/cathodyne.html

[AmberB]

When I wrote this post, I said that I tried to use a paraphase inverter circuit for the PI tube and it had a problem that I couldn't solve, so I pulled all the parts and rewired it to a cathodyne PI.  I'm finishing up the wiring on that now, and I think I know why I couldn't get the other PI circuit to work.  I wanted to put 1k resistors on the grids of the power tubes, the original amp didn't have them.  As I was preparing the tube sockets for the 1k resistors, I noticed that there are no 220k resistors from the grids to ground on this amp.  (The power tubes are cathode biased.)
I'm pretty sure that would cause the problems that I was having previously...
The Ampeg schematics generally call for a 270k resistor with their older 6L6 amps, so I'll go with those.
That was an "oops" I should have caught sooner...

[ALBATROS1234]

6SJ7 guitar amps sound killer . I would keep them on the input and boost the signal with the 6SC7s or change them to 6SL7s if you want to bias each triode differently. I guess it depends on the tone you want but alot of early guitar amps used 6SJ7 tubes. they have a unique tone that many people love.

[AmberB]

6SJ7 guitar amps sound killer . I would keep them on the input and boost the signal with the 6SC7s or change them to 6SL7s if you want to bias each triode differently. I guess it depends on the tone you want but alot of early guitar amps used 6SJ7 tubes. they have a unique tone that many people love.

Kind of too late for that now, I rewired the heaters on the sockets several months ago, and built most of an Ampeg type preamp setup.
I might try that next time if I run into another amp with all octal sockets.

[AmberB]

Here's my current rat's nest...

[bmccowan]

As rats' nests go - that's a pretty neat one!
On the 6SJ7 issue - that's the perfect excuse to go find another amp to convert. There are tons of them out there, PAs, Jukebox amps, old organ amps, projector amps, mono hi-fi etc. Sadly not super cheap on eBay anymore, but you get a chassis, two transformers, sockets, switches, etc for a little over $100. For the Grommes I mentioned, I intend two channels - one pentode - one dual triode, with the ability to blend the channels. I'll be looking for advice on here for sure.

[AmberB]

First voltage test, with just the 5U4 and the 6L6s in the sockets-after a bit of warm up-
B+ off the rectifier tube-405volts
screen grid node-345 volts
power tube cathode voltage (cathode biased)-37.7 volts
6L6 plate voltages-402 and 388 volts.
The power tubes are totally mis-matched, so that may account for the difference in plate voltage, or it may not...

With all the tubes plugged in...5U4, pair of 6L6GCs, three 6SN7s...and the speaker plugged in-
B+ off the rectifier tube-413 volts
screen grid node-357 volts
PI tube power supply node-325 volts
preamp tubes power supply node-281 volts
power tube cathodes at resistor-30.9 volts
power tube plates-407 and 406 volts
Apparently having the PI tube in circuit makes a difference to the balance of the power tube plate voltages, although the speaker being plugged in might also affect the plate voltages.

I plugged a guitar in and tested it briefly, the volume isn't as loud as it could be at full volume, probably just the 6SN7 tubes not putting out enough signal.  The amp barely gets into distortion.
I have 3 triode stages in the preamp, 2 before the tone circuit and 1 after, before going to the PI tube.  I have a volume control between preamp stage 1 and 2, and another between stage 3 and the PI tube.
I'm going to dig out my microphonic 6SL7s and see what they do in the circuit.

[bmccowan]

Have you drawn a schematic of your build? Voltage readings seem ok. I would expect this amp to be loud. You know the tubes are good?

[AmberB]

I have not drawn a schematic for the amp yet...I really should get that done.
I agree that the voltages seem to be about where they should be.
I tested the tubes when I pulled them out of my stash, and they tested good.  The problem with using 6SN7s is that they're a low output tube, about the same as a 12AU7.  They may simply not amplify the signal enough for the amp to get as loud as it should.  I was thinking about trying some 6SL7s in the preamp.  All of mine are microphonic, but they test good.  The other thought is to put bypass capacitors on the cathode resistors in the preamp.  That should help the preamp to put out more signal.
I have a buzz on the second volume control that I will have to find, but I'm going to try bypassing that volume control and see if the buzz goes away.

[bmccowan]

It took me years to figure out that I should be drawing up schematics for my projects. So I'm on a small pony rather than a high horse. Sluckey, PRR, Tubenit and other experts here will be able to provide much improved help with that done. I agree that 6SN7s are lower gain, and bypass caps will increase gain. But whenever I or somebody on the forum experiences lower volume than expected, it is almost always a wiring error, or a faulty component.

[PRR]

A PA amp Mike input, stock, should be more than enough gain for any healthy guitar player.

Fix what's weak/broke before you go souping-up.

[AmberB]

A PA amp Mike input, stock, should be more than enough gain for any healthy guitar player.

Fix what's weak/broke before you go souping-up.

I don't think anything is "broken"...it's not that the amp is really low output, as in something wrong low, it just doesn't get as loud as I would expect a 30 watt amp to get.  Kind of loud, but not as loud as my Fender Princeton.

I managed to draw up a schematic for the amp in it's current form, minus the power supply circuit.  The power supply has a new JJ cap can, 40, 20, 20, 20, all at 500 volts.

[sluckey]

Use 6SL7s for V1 and V2.

[DummyLoad]

http://www.nosvacuumtubes.net/product/rca-5691/

 


--pete

[PRR]

Awful lot of NFB around the output stage. No cathode caps. Total gain looks a little low, not as lively as most amps.

Does it distort easy?  not at all?

[AmberB]

Awful lot of NFB around the output stage. No cathode caps. Total gain looks a little low, not as lively as most amps.

Does it distort easy?  not at all?

It distorts slightly with the volumes pegged.  It's rather clean otherwise.
I don't remember where I got the feedback resistor value from, possibly an Ampeg schematic, or a schematic linked on a post here somewhere.

[AmberB]

http://www.nosvacuumtubes.net/product/rca-5691/

 


--pete

139 dollars per tube...I think not...

[AmberB]

Well, upon digging into my stash, it seems I have several 6SL7s to try, including 5 NOS GE tubes...now I'm wondering if those are the microphonic tubes I was using to test an Ampeg B-15 Portaflex amp a couple of years ago...

[John]

FWIW, I've had good luck using the silicone O rings over the octal tubes to dampen the microphonics.

[AmberB]

I'm going to have to see about getting a few of those silicone O rings.  I've heard of them, but have never used them.

[AmberB]

Swapping out the 6SN7s for 6SL7s certainly changed the way the amp works...
I still have a couple of little things to correct on it.  One of the original volume controls probably needs to be replaced, and I have a bit of hum to deal with.  The hum isn't bad, but it's there.

[AmberB]

I may have a bad power tube socket on this relic...I retensioned the contacts in the socket, now to test it again.
I burned my thumb taking the power tube out, that sucker was extra hot, and my thumb went past the shop towel I was using to pull the tube...

[DummyLoad]

http://www.nosvacuumtubes.net/product/rca-5691/

 


--pete

139 dollars per tube...I think not...

if you want octal tubes that were designed to be low microphonic and are, they are your ONLY choice! ;-) i gave up on building anything with octal preamps tube for that reason. i have a stash of about 10-12 that i'm saving for a rainy day if the B15N bug bites again.

--pete

EDIT: there is another octal that has respectably low microphonics - Raytheon 2C52 - 12.6V filaments though. very rugged tube and can still be found for $20-40 a copy. has a same pinning as the 6SL7/6SN7. 

[AmberB]

20 to 40 bucks per tube...much more reasonable...worth looking into.

[bmccowan]

Microphonics in tubes is a matter of degree. Not simply a case of microphonic or not. All tubes are microphonic to some degree, and circuits and amps are sensitive to microphonic tubes to a degree. Combos more than heads, high gain more than lower gain. I've built and repaired many all octal amps with 6Sj7s, 6SL7s, 6SH/Q/C7s, etc. with no more problems with microphonics than many 9 pin tubes. Octals get a bad rap. I recently finished an all octal version of a Matchless Clubman - no issues. I also recently bought a pair of NOS RCA 6SL7s on the Bay for $65. again no problem. 

[AmberB]

I've started working on this amp again.  I decided to use the extra unused triode on V1 for another guitar input.  I want to build this input differently from the Ampeg type input that is in the amp now.  It's another Ampeg input design, this time from the J-12-B.  The R-12-R uses pretty much the same input setup, except that it has 3 input jacks instead of the J12-B's 2.
 I want to use a bypassed cathode first stage on this input to see if I can hear any difference between this one and the unbypassed input that's more typical of Ampegs.  I do have to wonder why they used a 100mf capacitor for the bypass cap, and it looks like it's an electrostatic cap, not a lytic cap.  I'm guessing that the"mf" means that it's a .1uf, not a 100uf. 
I'll have to use the pair of 270Kohm resistors where I combine the output signals from the first stage to the tone control stage, but that shouldn't be a problem.

[bmccowan]

I'm guessing that the"mf" means that it's a .1uf, not a 100uf.

I think that cap actually is a 100uf (mfd) cap on that stage. If you look at the second stage the 25uf cap is also labelled with a simple M in the schematic I saw. There are now compact electrolytic 100uf caps with voltage ratings of 100 or 150v.
Glad you are back on this amp and will be interested to see what you come up with.

[bmccowan]

Further - I thought Sluckey had built this amp, but looked at his site and did not find it. But then when I did a search for the schematic - there is was - the RCA conversion. He will no doubt chime in on this amp. That will be a big help for this project.
http://sluckeyamps.com/RCA/RCA.htm

[sluckey]

100mf really is 100µF. Back in the '60s it was still common practice to use mf for microfarad. It really is an electrolytic. Using a huge cap on the first stage was done to help reduce filament hum. Sunn even used a 250µF!

Look at the M12 to see how they combined the two channels.

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_m12.pdf

[PRR]

...huge cap on the first stage was done to help reduce filament hum. Sunn even used a 250µF!

Also: one cathode-cap for two channels will leak bass, cross-talk. Turn Ch2 all the way down, you still hear the Ch2 input coming through _tumb-ba-bump. The bigger the cap the less the leaking.

Yes, two RC networks is another fix and off-channel leakage is not a horrible problem.

[AmberB]

Well, a 100uf at 50 volt 'lytic cap isn't very big physically, so I guess I'll have to try that.  :-)

[AmberB]

That RCA radio conversion is ridiculously cool!

[AmberB]

You know, the biggest problem I have with rebuilding this amp is not what parts to use for the circuits, but where to put them and how to arrange them on the tab strips in this point to point mess...
I had to pull out 1 of the tab strips next to V1 and put in a different one with more mounting tabs so that I would have a place to mount the extra parts...

[bmccowan]

You know, the biggest problem I have with rebuilding this amp is not what parts to use for the circuits, but where to put them and how to arrange them on the tab strips in this point to point mess...
I had to pull out 1 of the tab strips next to V1 and put in a different one with more mounting tabs so that I would have a place to mount the extra parts...

I certainly relate to that experience. I've rewired quite a few old PAs and that is a difficulty. Not just the number or caps and resistors but the fact that parts from the 40s and 50s were physically robust, whereas modern components are wimpy. The much heavier gauge wire used in manufacturing older components allowed them to be wired point-to-point without added support. Like you, I also add tag strips and single turret posts where I can to make some convenient connections. It helps to draw it up - I use DIY Layout Creator adjusting the components to be proportional in size to whatever components I'm using. It rarely works out perfect, but armed with that and a schematic of the intended circuit, I can proceed without getting too lost. Establishing a good ground scheme from the start also helps.

[eurekaiv]

...Not just the number or caps and resistors but the fact that parts from the 40s and 50s were physically robust, whereas modern components are wimpy. The much heavier gauge wire used in manufacturing older components allowed them to be wired point-to-point without added support.

I've been doing a bunch of PA and junker amp rebuilds over the course of this past year and this is absolutely the most annoying aspect of it. I started using stripped ptfe insulation on any part that was wired point to point and it helps a lot, but man, resistors now'days... the leads are probably 50% of what they were even in the 70s.

[AmberB]

I finally got around to making some more progress on this amp.  I had put aside for a while due to other distractions.  I built a second input channel for it using the unused triode of V1.  I originally had a gain control and a post volume control on the amp, I had to change that to just a volume control for each channel.  I only have 4 controls available on the amp and 2 of them are used for the bass and treble controls.  I used the volume controls on the output of the first stage, and tied them to the input of the tone amplifier using a 220k resistor from each volume control to the grid of tone driver triode.
I discovered that you can't use a 6SL7 for both the first stage tube and the tone circuit tube, it's just too much gain. After a bit of tube swapping to see what works the best without further circuit modification, I ended up with a 6SL7 for the first stage tub, a 6SN7 for the tone circuit tube, and a 6SL7 for the power tube driver/ phase inverter tube.  It still distorts a little sooner than I would like, so I'll probably try a 6SN7 for the P/I tube also and see what happens.  I tried the amp with a 6SN7 in first stage and a 6SL7 in the tone stage, but that didn't work very well.
The process continues...
I need to make a back plate to cover all the holes in the back of the chassis, then I can mount an impedance switch and the speaker output jack and have all the other holes covered.