Warm Transformers Then Oscillations When Auditioning Rft Ecc83 Valves

Discussion in 'The Workbench' started by RickyLee, Aug 1, 2016.

  1. RickyLee

    RickyLee Well-Known Member VIP Member

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    I have an '87 4104 (2204) that I have modified to have an extra gain stage. Amp sounds decent so far and was playing quite extensively last night with no issues. After shutting it down I noticed the power transformer and the output transformer were quite hot to the touch. I got concerned as I have never noticed any of my Marshall amps getting that hot, well the older JCM800 era amps anyways. This amp still has its original transformers but it has new filter caps installed. I checked and it is running the proper rated fuses for safety.

    I started thinking the heat could be the tubes, maybe the EL34's. I do need to load in a new set of EL34's as this has an oddball mix of one JJ 6CA7 and one Svetlana EL34. They are biased close with 2mA of each other while the 6CA7 is running a bit hotter and the 6CA7 is drawing 6mA through the screens while the EL34 4mA through the screens. Plate is 480V and I have the bias set at aprox 31mA/29mA shunt method measured across OT windings. Heaters are running 6.9VAC due to my high wall voltage.

    I do have a bit of hum which is audible if i turn the master volume up 8 to 10, which is normal for a high gain amp anyway. Last night I started rolling some different 12AX7's through it tp see if the hum would clear up a bit more. That is when I had some oscillations happen. The oscillations only happened when I put an RFT ECC83 in V1. I initially had Chinese 12AX7's in it. I put an RFT in V2. Then when I added the second RFT in V1, that is when the helicopter noises kicked in. This happened with no guitar plugged in and master volume off. When I turned the gain down to 0 the oscillations stopped. Tried a different RFT in V1 and same problem.

    So I am now wondering if the oscillations are due to my high gain circuit and the RFT's possibly having much higher gain than the previous tubes I had in it?

    As for hot transformers, I am going to load in a new matched set of EL34's later and see if that helps. But as for the oscillations, I was already thinking along those lines when I discovered the transformers being hot. I was thinking that there could be oscillations happening that were not audible but causing the transformers to work harder. Then when I stuck the RFT's in there I did here oscillations. So that was a trip for sure. I do not have a scope to check for this. And is there another way to check for oscillations that are not audible?

    I might go ahead and increase the snubber cap across the phase inverter plate resistors from the stock 47pF to 100pF for the time being. I already have at least one snubber cap in the preamp, on the second gain stage if I am remembering correctly.

    The RFT's I have were a set of four supposed NOS. All of them did the same noise when I tried each one.
     
  2. charveldan

    charveldan Well-Known Member

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    Sounds like the RFT have a weird transconductance thing with ur amp.
     
  3. danman

    danman Active Member

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    It sounds as if the amp does not agree with the RFT's. There are some measures that could be implemented that may help with the oscillation, as may adjusting the lead dress. I wonder if the extra current draw from the added tube is causing the warm PT or is it possible that you just never noticed it in the past?
     
  4. RickyLee

    RickyLee Well-Known Member VIP Member

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    Whoah Nellie . . . . Hold the Horses.

    I was just looking inside my 4104 at that phase inverter snubber. Something did not seem right. Turns out that this 47pF snubber is in a different location than I am used to seeing. Instead of being strapped across the phase inverter plates on the signal side, this one is strapped across the phase inverter to power amp legs on the outgoing or down stream side of the two coupling caps. I would imagine this would have somewhat the same effect, but would it not do more to the audio signal being placed after the coupling caps? I was considering just adding a second 47pF onto the top of the board instead of replacing that 47pF. What would be the effect if I just placed this second 47pF in the more common spot across plate resistors and leave that other one?

    Or should I yank that existing one and just go with a 100pF across the plate resistors?
     
  5. RickyLee

    RickyLee Well-Known Member VIP Member

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    Loaded in RFT's in V1 and V2. Still same old EL34 beaters in power amp. I thought the problem was gone and these preamp tubes sounding really good. Then I unplugged the guitar cable from the input to test and helicopter sounds were right back. Still there if gain is set to 0. But they go away when master volume is off or 0. I will load the chassis into the head shell and see if that cures it.
     
  6. RickyLee

    RickyLee Well-Known Member VIP Member

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    OK, something else to note. Noise is still there with chassis shielded in head cab. BUT, this noise goes away if I switch on the gain pot bright cap which I have on the gain pot push pull switch. Odd as engaging the bright cap increases gain quite a bit. I went over my lead dress last night and tried moving wires and such to see if I could improve noise. But this amp is actually pretty quiet for a high gain amp. It is basically still the same Marshall 4104 layout but I added shielded cable in quite a few spots.

    So I think the issue is in that area on the push pull switch with that open bright cap acting as an antennae.
     
  7. danman

    danman Active Member

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    Pulling the cable from the input should ground the grid. Anything wonky with the input jack or solder connections?
     
  8. RickyLee

    RickyLee Well-Known Member VIP Member

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    A 100pF snubber across the V1A plate solved it.
     
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  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    Yeah, it has me thinking something is not 100% somewhere. But then, this only happened with the RFT's. Maybe just the different capacitance characteristics of those RFT's? But then, the other thing is it goes towards what you mentioned as it only happens with the cord unplugged.

    WAIT . . . the grid of V1A does not get grounded due to there is a 47nF cap between my input and the grid.
     
  10. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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    After reading this thread, wow. RickyLee, you really seem to know your shit when it comes to amps...nice! Hope all is solved and I hope I learned a bit here???
     
  11. RickyLee

    RickyLee Well-Known Member VIP Member

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    I just got done playing it for a good long while. Left the RFT's in V1 and V2. Still have a used China 12AX7 in the phase inverter V3. About half way through the tweaking session of experimenting with different coupling cap values in a key location, I said WTF and grabbed a new set of Ruby EL34 BSTR's I had in the stash and loaded them in. Immediately it was like I turned a few notches on a Resonance circuit as the bottom end got bolder and the volume a bit bigger as well. That oddball match of a JJ 6CA7 and Svetlana EL34 was just a junker grab when I was first working on this amp a good year ago. But the mix of those two valves did yield some very sweet crunch. But in the end, with the transformers running warm and those old EL34's screens passing over 6mA at idle, I figured I better quit while I was ahead there.

    So after the new Rubys were installed (they are only each passing 1.8mA on the screens at idle and biased up within a mA of each other) I played it for quite awhile, then took voltage readings to write down from every key area of the amp including the audio signal levels up to the entrance of the master volume pot. This amp has just a slight bit less punch and balls of a standard 4104/2204 due to all the tweaks I did to the preamp ( I had a bit less AC voltage/audio signal coming into my master volume than I would see from my '83 2204). But it has an extreme amount of extra usable gain. Later on I am going to add in a second channel for a clean. Not an actual channel, but just a bypass with its own gain control that can be footswitched with a relay. Would like to have an FX loop as well. But regarding the warm transformers, they are still warm but did not seem as hot as they were last night. Should not be an issue. I am just remembering many of my old Marshalls actually running surprisingly cool on the iron.
     
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  12. johan.b

    johan.b Well-Known Member

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    The bright cap on the gain pot reduces the impedance in that part of the circuit. It could be that the high impedance of the gain pot without the cap simply picks up noise that can't get through with cap engaged. ..you could possibly confirm this. If the noise/oscillation goes away when the pot approaches min or max (with cap disengaged), i would expect high Z in a sensitive area to be the problem.
    ..and if so, you could probably work around the problem placing a100k resistor on gain pot viper to ground..It would mess with the curve and cut a bit of lows at max gain, but everything has a price. ..
    j
     
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  13. RickyLee

    RickyLee Well-Known Member VIP Member

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    Yeah, I have been learning to compromise a bit here and there with many mods for sure LOL.

    At this point, it is cured with the 100pF snubber across the first gain stage plate resistor. A good compromise as I did not gave to lose any gain or alter the EQ response.
     
  14. HAMPAMP TUBE AMP SERVICES

    HAMPAMP TUBE AMP SERVICES Member

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    If you are interested in dropping the filaments down a bit you can try wiring a "bucking transformer" to your mains input. I use one of these with all vintage amps made to run on a lower line voltage. http://www.angelfire.com/electronic/funwithtubes/Buck_Boost.html instead of a 12v transformer you can try a 5v, 3A filiment transformer. The only problem is that it would drop your B+ a little as well but would hardly be noticed but if your filaments are running high, the B+ is probably high as well
     
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  15. RickyLee

    RickyLee Well-Known Member VIP Member

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    Actually I made up one of those from a Radio Shack transformer quite some time ago. It drops my wall voltage from 125VAC down to around 111VAC. I need to install a switch on it and use the extra tap to give me a slightly higher option of around 117VAC. But yes, they work great.
     

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