Oscillation in 5E7 Clone

[Klugetone]

Hello all. Been away from homebrew amp-building and Ampage for 10 years or so.
Playing out with the band constantly during that period left little time for tinkering with the hardware. My old DIY amps served me well for many many shows. Recentlly quit the grind for awhile and decided to build something new. Settled on a fairly stock 5E7-type circuit. Got it built and it sounds great, for the most part. Tested it out using the 4 ohm speaker pair in my Blues Deville. Worked like a charm. Very little noise, decent tone etc. Then I connected it to a single Eminence Screamin' Eagle at 8 Ohms in a sealed cabinet and it immediately started howling at around 300Hz with no input and volume controls at zero. the amp is about 5 feet away from the speaker. Sometimes, it won't howl immediately. Just nudge either volume control though and off it goes. Could be something as simple as lead dress but nudging the wires around aren't making much difference. Can't detect anything overly microphonic either. Interesting case. Back to an 8 ohm paralleled speaker pair this time and all is well. Could the damping effect of parallel speakers keep this beast under control and it loses that with a single speaker? The amp was built using iron from Triode, Chassis from Mojo, and NOS tubes, resistors, sockets and stuff from my antique radio parts inventory. Made the eyelet board an exact replica of Fender's. Wiring is ok. All caps are new mylar and filter caps are new also. Voltages across the board are well within tolerances including bias on the 6L6GCs. B+ is 418 volts from a solid-state rectifier. Any ideas?
Thanks In Advance!
Dave

[Jazz P Bass]

Amp oscillation

Try reversing the output transformer primary leads where they attach to the plates.

[Chuck H]

+1

Jazz didn't explain why...

At 4 ohms there is less voltage in the feedback loop than at 8 ohms. If your feedback loop is positive instead a negative (it should be negative) the added voltage in the loop when plugged into 8 ohms could be whats causing the oscillation.

Does your presence control work properly now?

Chuck

[Klugetone]

...slaps head with hand...hard!

Reversed leads and the howl is gone and yes, the Presence control works properly now! Had completely forgotten about that detail. Thanks so much guys!
It seems my mind has been stuck in the digital world for far too long. Feels good to get back to my roots.

I am happy to see this group is still going strong and the technical expertise you all have is even better than ever! Had some fun years ago. Just might get a case of GAS again after this latest build.

Thanks again
Dave

[Klugetone]

Follow-up

Oh I gotta mention that after making the corrections to the amp wiring and hooking it back up to the Eminence Screamin' Eagle, it sounds absolutely fantastic!! It is a very good match for my purposes with the 5E7. Need pristine, glassy cleans up to a decent volume level. Got that big time! Plugged in my pedal board and fell right off my chair. It is perfect for my small-club gigs! Just need top build the combo cabinet now. But first an important decision! What knobs to use?? I have a lot of pointer knobs from my other interest in antique electronics...See photo...


Thanks again!

[Jazz P Bass]

Positive Feedback

+1

Jazz didn't explain why...

At 4 ohms there is less voltage in the feedback loop than at 8 ohms. If your feedback loop is positive instead a negative (it should be negative) the added voltage in the loop when plugged into 8 ohms could be whats causing the oscillation.

Does your presence control work properly now?

Chuck

Thanks for the explanation Chuck.
I had a hunch it was the FB, just could not take it farther.
It's possible to see the FB react if you disconnect the loop & then reconnect it while observing the output waveform.
If the the output sine wave decreases, it is set up properly for negative feedback.
Conversely, if the output increases, you have a positive loop.

[ThermionicScott]

Klugetone, that's exactly what happened with my 5E7 build. It behaved fine with the stock load, but the Presence control didn't work, and it howled when I plugged it into a 4-ohm speaker cab. That the Presence control didn't work (despite rechecking my wiring several times) should have been a clue!

- Scott

[Klugetone]

... That the Presence control didn't work (despite rechecking my wiring several times) should have been a clue!

- Scott

Scott, I know how that feels! My presence control didn't work either but I guess I figured it was because I had just started the tweaking process with the new build and hadn't done anything with the NFB values yet. Now I don't have to! Works fine for my tastes. Must have just got lucky on my earlier builds (5C3, 5E3, and a Pro-Lux 6G3) put OT wires to the right tubes. Very happy with the way the 5E7 sounds now. Not gonna mess with it too much more for now. Don't want to make it more of a Kluge than it already is!

BTW, I used the Fenderish 30-35 watt OT from Triode as a test case to see how well their iron works. So far it seems to be a good performer and value (even has 4, 8 and 16 ohm taps). We will see how it works during 4 hour gigs. Just gotta finish the cabinet now.

Thanks!
Dave

[ThermionicScott]

Well, the 5E3 is a gimme -- there's no chance of turning a negative feedback loop into a positive one.

It sure is a fine-sounding circuit. Apart from adding adjustible negative bias and a cathode-bias switch to mine, I haven't tinkered with the circuit.

- Scott

[MWJB]

"Voltages across the board are well within tolerances including bias on the 6L6GCs. B+ is 418 volts from a solid-state rectifier. Any ideas?" Which PT did you use, 418v with a SS rectifier seems unusually low (320-0-320VAC -ish)?

[Klugetone]

[QUOTE=MWJB;158264 Which PT did you use, 418v with a SS rectifier seems unusually low (320-0-320VAC -ish)?[/QUOTE]

Have a Triode PT #40-18005 - Rated at 420 VDC @240 mA. Also found that using an 82 ohm, 7 watt resistor inline with B+ kept the voltages close to original nominal. Wasn't crucial though. Otherwise they would have been somewhat higher at about 440 VDC with my actual current draw.


Thanks!
Dave

[MWJB]

440v isn't particularly high, your amp won't be drawing anyything like 240mA at idle, so voltages will come in somewhat over that. Try it without the resistor (& rebias) & see what you think.

[Klugetone]

440v isn't particularly high, your amp won't be drawing anyything like 240mA at idle, so voltages will come in somewhat over that. Try it without the resistor (& rebias) & see what you think.

When I originally tested the build, it didn't have the resistor and thought the tone to be a bit harsh and brittle. Of course, that was before I tweaked a few things such as an excessively high filament voltage (7.2 vac), dialing in the power tube biasing and before I discovered that the 12AX7 in the PI was very weak on one side. The PT was designed as a general replacement in Fender amps with reverb and/or tremolo also which would provide more current draw on both HV and Heaters. I will most likely remove or reduce the resistor value and see how it sounds. I am using 450 volt filter caps so don't want to push them too much though. A little more clean headroom would be a plus for me!

Thanks!
Dave

[MWJB]

Main B+ & screen supply caps should really be 500v+ rating, you wan at least 50v headroom over the voltage rating.