Epiphone Futura RVT channel 2 power loss

[Coldhand75]

I have a cool old Epi Futura tube amp that I restored electronically earlier this year.    It was fully functional (including reverb and tremolo) and sounded great on both channels until I had to remove a couple preamp tubes 10 days ago to work on the cabinet.   When I reinstalled the tubes, the amp powered up right away and channel 1 was great.   Channel 2, however, is only running at about half power.   I checked the preamp tube sockets (for 3 6EU7s) and found a broken ground on one which I repaired.   Still only half power though the tone controls, reverb and tremolo are still working fine.   I tried changing out the preamp tubes (I have another set).  Still half power.  There is a brief power/volume surge on Channel 2 when I switch from "On" to "Standby".    Any idea about what might be going wrong?

Here's a little background:    This amp was in poor condition in February when I got it.   First thing, I replaced all six filter caps with new F&T caps.   Channel 1 came back to life loud and clear.  Virtually no hum.   I also had to replace two missing pots, two jacks and a few resistors.   Then channel 2 came back at full power and worked great until 10 days ago.   This amp has two clean channels -- channel 1 with bass and treble only and channel two with the same tone controls plus reverb and tremolo. The effects are tube- and trany-powered.  All the tubes in this amp are new including three Tung Sol 6EU7 preamp tubes, a Tung Sol 12AU7 (phase inverter?) and two Mullard 7591A power tubes.   

[shooter]

schematic?
Doug's library has a bunch, didn't see Futura, but it could be list by a # like 32, 33...


does wiggling the tubes in they're socket change anything, like crackles, volume changes....?

[dude]

People here are going to want to see a schematic…

[Coldhand75]

If I've done it correctly the schematic is attached.   The schematic is for the comparable Gibson amp.   I used this schematic when I working on the amp earlier this year and there were only a couple obvious differences (e.g., no mid-range tone control on the Epi).   I set and re-set the tubes and wiggled them.   No change in half power on channel 2.

[shooter]

Still only half power though the tone controls,

how did you figure this ^


check the input jack, cables, shields


EDIT:
what's the end-game for this amp?
IF it's a players amp, consider re-building the channel
IF it's a Commodity investment, find the bug.


do a DC volts check, plate n cathode, CH1 VS CH2 - NO signal

[Ed_Chambley]

Did you check your voltages?

[tubeswell]

Here's another 'Epiphone Futura' schematic

[Ed_Chambley]

We are asking?

[Coldhand75]

Shooter & Ed:  Thanks.  I'm having trouble using my old Tandy analog meter to measure DC volts.  I may be reading the scales wrong, or perhaps taking voltages off the wrong pins.  This is what I found:  The second 6EU7 preamp tube shows almost exactly the same voltages coming off the side A and B anodes.   A is Channel 1; B is Channel 2.   The third preamp tube is different.   It shows a slightly smaller voltage coming off anode A and a 20% higher voltage coming off B.   Channel 2 is currently running at about half power -- half the volume output as I hear it.   I'm new to this, so I admit I may be measuring the wrong things.

This amp is not for resale.   I'm restoring it for my own use.    I checked all the jacks and connections, wiggled the tubes, checked shields, etc.   Nothing so far affects the lower volume output on channel 2.

The Gibson GA-30RVT schematic is closer to this amp than the Epi schematic.   As I understand it, Gibson changed the Epi Futuras in the mid-1960s and did not bother to update its schematics.   This is a nine-tube Epi Futura.   The Epi schematic is for an earlier version with eight tubes.   Thanks anyway.

[sluckey]

I have a cool old Epi Futura tube amp that I restored electronically earlier this year.    It was fully functional (including reverb and tremolo) and sounded great on both channels...

A bit late now, but this would have been a real good time to measure the voltage on every filter cap and every pin of every tube. Record and keep in a safe place. You may need this info sometime in the Futura. (pun intended    )

The sparse info you have provided so far points to a problem in the CH 2 preamp, so it's probably a simple and inexpensive fix. But without a schematic, we're just shooting in the dark.

That Tandy meter is probably not up to the task of troubleshooting voltages in a high voltage, high impedance tube amp. At best, the sensitivity will be 20,000Ω/volt. At worst, the sensitivity will be only 1,000/volt. Neither is good for measuring grid or plate voltage.

Continue searching for a proper schematic, or attempt to draw it yourself, or take it to someone that can do it for you.

Meanwhile, back up 10 days when the amp was working fine. Now go over everything you did to the amp when you pulled the two tubes, including things you may have inadvertently or accidently done to the amp. Shouldn't be too hard unless a hammer or power drill was used.   

[Coldhand75]

Thanks to everyone for your comments and suggestions.  Much appreciated.  FYI:  the Gibson schematic (GA-30 RVT) that i posted above IS the best schematic for this amp.   The Gibson & Epi amps were virtually identical in mid-1960s except the Epis lack a "Mid" tone control.   I successfully used the GA-30-RVT schematic to replace damaged pots, caps and resistors and the amp performed wonderfully when I did.   Another member of this forum uses the same Gibson GA-30RVT schematic on his Epi Futura RVT.

I tested the DC voltages on preamp tubes 2 and 3.  They were very close to what the schematic called for:   2nd 6EU7:  180vDC on both A&B anodes vs. 170vDC on schematic; Same for B side of 3rd 6EU7.   Side A of 3rd tube anode came in at 240vDC vs. 221vDC on schematic.   So, it appears to me the voltages coming off the 2nd and 3rd preamp tubes are close enough to nominal to say that this section of the preamp stage is functioning as intended.   I agree something is still wrong with the CH 2 preamp stage.   I can't pinpoint it so far.

I thought back about what changed when I removed the tubes (over 10 days ago) and replaced them, but CH 2 was at half volume.   Amp was treated gently.  No power tools near it.  The ground wire to preamp tube socket No. 1 was broken, so I fixed that.  The socket also wiggled around on the chassis, so I replaced the broken aluminum staple holding it in place with a small steel nut and bolt.

[shooter]

what's the cathode VDC for V1B, V2B?

[Coldhand75]

I'm getting 1.4 vDC on both the V1B and V2B cathodes.

[shooter]

looks like the DC side of things is good, time for an Oscope to probe the AC side of things

[Coldhand75]

Shooter, thanks.    I think I found the culprit:  a poor pin connection for the OA2 tube (the oscillator) in the tremolo circuit.  CH2 runs through the tremolo and reverb circuits; CH1 does not.  When I giggled the OA2 in its socket, CH2 went to full power albeit sometimes with a very loud hum.   I played around with the tube pins and eventually got full power on CH2 with minimal hum.   I'm puzzled.   I giggled ALL of the tubes in previous tests and no help.   Today is different somehow.   All I did today is get the vDC readings off of V1B and V2B.  Anyway, I'm glad its up and running again even with a touchy OA2 tube connection.