Problems with EF 80 micro push pull

Blueslover88

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Hey guys,

Built Rob's 5e3 micro this time with two EF 80 in push pull for power tubes. I used the musical supplies site for the iron. Both the 190-0-190 pt and OT5PP output trans. I only connect the black 0 ohm and 8 ohm wire to a single output jack. The amp sounds okay, not as good as I want yet or as good as my 5e3 micro with the 12au7 power tube. I also get a wierd squeel/hiss (oscillation?) When any of the three controls hit max. Turn them down from max to nine or less and it goes away. Does it with or without guitar and swapping OT wires to the plates had no change. My voltages are a touch lower then some other EF 80 builds ive seen on here but they used the 200volt toridial PT and I'm only using 190 regular PT. Any more infor you need to help me out.? All built to specs on @robrob site thanks for help in advance
 

2L man

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There are many possible reasons for amp to oscillate. Sometimes oscillation happen very high over 20kHz frequency from what we hear only lower harmonics.

Input jack wiring is where I have found my mistakes few times.

If it is not difficult unscrew PT or OT and slowly turn it at least 90 decrees and then test same turning as far as wires allow. PT and OT can interact electromagnetically.

You can "ground" one at a time all control grids of cathode biased triode stages. I just tack solder a short piece of wire. If it quit oscilation it is that stage or previous towards the input.

Moving wires might change oscillation and lead you to the reason.

Cutting wires as short as possible can help.
 

King Fan

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Your amp doesn't sound as good as you'd like, and it squeals at max on any control, right? I guess that could be one issue, or two. Lotsa causes either way. (FWIW if your amp doesn't have NFB, swapping OT leads isn't a cure).

You note the build is 'to spec' and voltages are just a bit low, but in troubleshooting threads, the two most helpful items tend to be good pics and a complete voltage table.
 

tubeswell

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You need to supply us with more information to help you - otherwise its too much guesswork for us.
Schematic
Voltages
Hi-res pics of layout
 

Blueslover88

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You need to supply us with more information to help you - otherwise its too much guesswork for us.
Schematic
Voltages
Hi-res pics of layout
Here is the pics if you need closer let me know. I was poking around when the squeel is happening and found the wires from the 68k to v1 pin 2 is what's causing it. Should I reflow or possibly use shielded cable?
Voltages are as follows
V1(12ay7):. V2(12ax7):
Pin1-101vdc. 1- 119vdv
Pin3- 2vdc
Pin6- 105vdc. 6- 148vdc
Pin8- 2vdc

V3(ef80). V4(ef80)
Pin1- 7vdc. 1-6 vdc
Pin7- 253vdc. 7- 252vdc
Pin8- 239vdc. 8- 236vdc

B+1= 236
B+2= 205
B+3= 156

Anything else you need let me know thanks souch
 

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Blueslover88

TDPRI Member
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Your amp doesn't sound as good as you'd like, and it squeals at max on any control, right? I guess that could be one issue, or two. Lotsa causes either way. (FWIW if your amp doesn't have NFB, swapping OT leads isn't a cure).

You note the build is 'to spec' and voltages are just a bit low, but in troubleshooting threads, the two most helpful items tend to be good pics and a complete voltage table.
I replied and posted more information. Let me know what you think, this time around the voltages look a little better than when I first measured. In the pic I posted my plastic poker is pointing at the wire that changes the squel. It's wires going to pins 1 and 2 on v1. Shielded cable maybe help???
 

tubeswell

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Voltages might be okay - but can't really tell whats going on with your output tube operating condition without information about method of bias, resistor values etc.

Looking at the pics of your layout, there are some improvements that could be made to improve the situation.

Remove the 68k input resistors from the eyelet board and relocate to the input sockets, and run shielded cable all the way from there to the input stage grids.

Also use shielded cable from the vol pot wiper to the following stage's grid.

Also put 10k grid stoppers on the grid pins of the EF80s and 470R screen grid stoppers (right at the screen grid pins) - to snub that as a source of oscillation.

Shorten the signal wire runs more
 




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