Need advice and/or suggestion re: Tube Power section mated to SS preamp

[smackoj]

hi fellow tone chasers. I have been trying to mate a stand alone SS guitar preamp (Peavey) with a tube output section. I did have it working pretty well with a 35 watt homebuilt Proluxe 6L6 amp but with the 12ax7 preamp section in the Proluxe it has too much (harsh sounding) gain.  I swapped out the preamp and PI tubes with lower gain 12AU7 and that brings it into a useable range but I'm hoping to improve it.  I then found an old radio chassis and tried connecting the input signal wire directly to the grid of the PI (6sl7 variety) and that doesn't seem to work at all. No signal passing to the output. I probably don't have something hooked up just right but I need some advice. 

do any of you amp gurus have a schemo of just a tube output section that I can plug a preamp output directly into without having gain stages to go thru first?  even if it just uses one triode preamp tube with low gain if it has to have that in order to properly run/control a tube PI. I am thinking that to run a pair of 6L6s it must have a PI??

all thoughts welcome

smacko jack

[MakerDP]

Quite a few to choose from here...

http://ax84.com/corepoweramps.html

I've built the 20W-PP mated with a Fender-ish preamp and it's a beauty.

[HotBluePlates]

Proluxe Schematic

Pick the input to the phase inverter. All old power amps were just the phase inverter/driver, output tubes and power supply. The catch is you've got to find the actual input to the phase inverter.

In the schematic above, the "useful input" to the phase inverter is actually the grid of V2A. This is the "pre-gain" stage for the split-load inverter, and a point where you can reasonably expect 1v of drive signal to result is full power output. 1v of output is probably a reasonable amount to expect from the output of a preamp.

Now don't just connect to the grid! The grid needs a ground reference, meaning a resistance to ground, across which to develop the input signal. For this circuit, that's the Volume pot.

If you're using solid-state, you might not be able to assume a lack of d.c. on the output of the preamp, so you also need a coupling cap. I'd attach my "Input jack" to the end of C2 not connected to the Volume pot (but also not connected to the tube plate as in the schematic), then have the Volume control (maybe turned around in "normal" fashion), the the grid of V2A (probably without the tone control).

All of the above mimics most small-mid-sized mono power amps of the tube hi-fi era.

[smackoj]

great information. thanks mucho amigos.

smacko     

[eleventeen]

When you are talking about changing from a 12AX7 to 12AU7 you are talking about the extremes in tube gains across that family, from high to low in this case. In the middle are tubes like a 5751, 12AT7, and 12AY7. Many people don't have any 5751's or 12AY7's hanging around. But see if you can dig up a 12AT7 to swap in for your first preamp tube and see if that does it. 

[tubenit]

With a passive effects loop prior to an LTPI.  Just plug into the return jack going straight into the LTPI.

(Essentially what HotBluePlates said)

With respect, Tubenit

[smackoj]

No luck tonight. dog tired so I'll give it another go tomorrow. thanks for all the input. I have the concept down, just must have a wire in the wrong place?

 

[HotBluePlates]

I have the concept down, just must have a wire in the wrong place?

Probably as simple as that.

The signal from your preamp will be a voltage signal. It needs a resistance across which to develop. In a typical Fender amp input jack, that's a 1MΩ resistor.

Preamp tubes almost always operate by accepting a voltage input. Same as above, that voltage needs a resistance across which to develop. That's usually a resistor from grid to ground, but could also be a volume pot at a tube's grid (which is itself a resistance from grid to ground).

You probably know this, but you shouldn't just connect a "hot" wire from the preamp to the power amp; there has to be a ground return to complete the path across that input jack/grid resistance.

[smackoj]

Eureka! It is now running smooth and 'really' quiet which was my goal. I haven't been able to turn it up to a lead guitar level because the grand daughters are both asleep.....but later I'm gonna crank it UP!

The major problem I was having is that although I tested the SS preamp into the Proluxe that is not the tube output section I intended to use as a stand alone tube output section.  I had an old radio chassis that I had previously converted to an early Valco Tweed design using a 6SQ7 preamp tube and a 6SC7 P.I.  I had already converted the P.I. to the 6SL7 but I had disconnected all the pt to pt wiring because it was a hodge podge wiring job so basically all I had was a bare chassis and a nice bunch of old bakelite octal tube sockets.

So, when I started back into this stand alone output section I wanted to use 6L6s for more power and either a 6SN7 or SL7 as the P.I. and I had to rewire the P.I. socket. As HBP discerned, I did not have the 'input' side of the 6SL7 wired correctly and I did not have a blocking cap and some variable resistance on the input to the P.I which made for zero, zip, nada signal to the output tubes.  So, after I got the excellent information and schematics to look at from all you helpful folks, I finally got the right combination of voltages, wiring layout and proper connections from the P.I. to the 2 output tubes produces a nice, clear, quiet signal out the speaker.

I'll try to load a pic or two later and give a more in depth rundown on how it works together with the Peavey ss preamp.

 

[HotBluePlates]

... As HBP discerned, ...

You figured it out, we justed faced you in the right direction.

Congrats on getting it working!! 

[smackoj]

Update on the stand alone tube output section. working at a very useable and nice sounding level, BUT one problem has sprung up.  When I first fired it up with the current limiter, it seemed like the voltage on the 6L6 plates and the 6SL7 plates was too high. Like 450 range with no load. So I added some big wattage resistance to see if it would play nice at lower voltage? It didn't like that and sounded weak and distorted, and the resistors were getting real hot too.  So, as I removed resistance the sound and output came into 'sounds right' range but after it warms up (3-5 mins) it starts intermittently making a pop noise. I only notice it on the lower register notes. I checked all the solder joints and couldn't find anything loose. I switched speakers and it still does it. So, my best guess is that one or more tubes is arching from too many volts? I am going to also try a beafier 5U4 recto tube because I just read that a 5Y3 may not supply enuf current for the bigger output tube types and I have been testing with a 5Y3 recto.

I'll update later today.  thanks amigos.   

[HotBluePlates]

...  When I first fired it up with the current limiter, it seemed like the voltage on the 6L6 plates and the 6SL7 plates was too high. ... as I removed resistance the sound and output came into 'sounds right' range but after it warms up (3-5 mins) it starts intermittently making a pop noise. I only notice it on the lower register notes.

Is this going on while still plugged into the lamp limiter? But the bulb is only glowing dimply?

If yes to the above, STOP. Unplug the amp from the limiter and run it on regular power.

The only function of the limiter is to diagnose short-circuits while preventing damage. Voltage measurements and playing performance can't be reliably assessed while using the lamp limiter. That's done while bypassing the limiter and/or plugging into regular power.

[smackoj]

"is this when it is on the limiter"?  NO, I'm on full power now and it sounds very good except for the popping sound.  I tried a 5U4 recto but that did not stop it.  I don't see any 'arching' in the tubes but I'm not sure that means one or more aren't arching?  I'm suspicious that one of the power filter caps is going bad or maybe I have a loose wire/bad solder joint?

Any other ideas I can investigate?

thanks   

[HotBluePlates]

I'd vote for the loose wire/solder joint. Note that the "loose wire" could be loose tube sockets if you scavenged them from something old.

An arcing tube would either be apparent as flashing inside the tube envelope, or in the form of burned carbon track between tube socket pins. But in any home-brew project, you have to be suspicious of your own work first (at least in my builds, non- or odd-functioning always comes down to some error, omission or slop on my part).

[smackoj]

Update late Sunday afternoon Jan. 4th.  I tried a re-solder on all the joints, replaced the 80 uf cap on the power supply and moved everything I could move. Still popping after a minute or two of operation. Seems like it starts to pop more frequently as it gets warmer.  It also now makes the same pop sound when I switch off the standby switch.  I did not chop stick poke around with it on yet, so maybe that will turn up the culprit?  It sure sounds to me like it is a high power pop like it must be coming from around the power filter caps or one of the wires with B+ on them.

 

[smackoj]

Tuesday eve. and still trouble shooting this stand alone power section.  I have re-soldered everything twice and triple checked for wires touching etc. with no change. I have been running the amp with the lights off and visibly watching for arching or changes in the tubes.  It only does this after about one or two minutes run time and everything is charged and warm. So, watching in low light and the popping starts, one of the two power tubes loose its' blue glow.  Any ideas what that indicates? I have tried different tubes, same results.

thanks, jack   

[HotBluePlates]

Any ideas what that indicates?

Nothing definitive.

Blue glow in common available tubes can fluctuate with any signal. So all it might be telling you is the output tube was amplifying the "pop."

The best troubleshooting method is one that rules out possibilities, not one that seeks to confirm a hunch.

Perhaps start by disconnecting the power amp from the external preamp (rules out the preamp and connecting cable); if the pop is gone, you know the power amp is good and the preamp/cable become suspect.

If the pop remains in the power amp, disconnect the circuit somewhere in a mid-point. Might be some gain stage or maybe the coupling caps to the output tubes.

Rinse, repeat until you've isolated the section of the circuit where the pop starts/stops.

[smackoj]

At last, and after probably 4 or 5 additional hours testing and re-doing this amp, it's finally working correctly. I believe the worst mistake I made was that using the vintage dark brown tube sockets, made seeing underneath where all the wires go very hard. The sockets did not sit the same direction on the chassis so the orientation of the pins was not the same. I got the power wires on one socket on pin 4 and on pin 6 of the other power tube socket. Not good at all. So finally I got both power tube sockets wired correctly and the amp sounds very nice, good clean power and very low noise. And the power tubes, recto and phase inverter all survived the multiple start-ups with the bass ackwards wiring which is a BIG plus.

thanks for all the help amigos.