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Old 6th November 2014, 09:00 PM   #1
Green77 is offline Green77  Sweden
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Default Help with new pp kt77

Hello all!
Just finish, i hoped, a new amp with kt77 output tubes. It's build on a pcb from ebay with 12ax7 (i use 5751) input to 12au7 to kt77 pp.

I use uf4007 bridge to 100uf - 4H - 100uf (B+ here) - 2.1k - 50uf (pi here) - 150k - 50 uf (5751 here).

Bias set to 50mA per kt77 (fixed bias). All dc looks ok with dmm. No NFB connected. Opt is edcore 50w 8K. Last year i build a exactly the same pcb and setup but with el34. And that one worked directly.

When i hooked up speaker a loud hum and hiss was coming out. No music at all.
I swaped preamp and pi tubes but no change, and both sets works in my other amp. But when i remove the phase inverter tubes the hum and hiss dissapere.
I cant find the problem! Since booth channel have the fault, could it be the PS?

Regards //Daniel
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Old 6th November 2014, 09:33 PM   #2
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Put PCB from old amp to new using only end stage.
If there isn't sound, measure +U on plates to all end tubes, +U on the middle point of OTr, +U DC on every power point in PS, U AC on the AC points of PTr. If rect. is from diodes bridge, +U on the bridge must be U AC X 1.3. Maybe there is faulted uf4007 or el cap in PS for section Ph. Inv. or preamp.
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Last edited by azazello; 6th November 2014 at 09:47 PM.
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Old 7th November 2014, 06:50 AM   #3
Green77 is offline Green77  Sweden
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Thank you. I will messure tonight.

Regards //Daniel
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Old 7th November 2014, 07:21 AM   #4
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OK! Be carefully, when measure High Voltages in PS. By the way, check U under common cathode resistor /if there is from both cathodes to ground/, U/Rcathode=about 100 ma /acc. nominal 2X50ma in both tubes/. If there isn't resistor and cathodes are conn. to ground, You can measure the same results /-U/ on the first grids of end tubes. Maybe modes are OK, but check.
Solder again all solders in PS and output stages.
Check all ground points and their connect to ground cord.
Check all wires between ph. inv. and end stages....more important, ground wire between them.
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Projects: OTL-OCL 6AS7 Gen. Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, E84L Siemens, E236L Telefunken, headphone ampl. OTL Loftin White 6AS7 RCA,... PPs 6005 Gen. Electric, EL34 Mullard, 6L6....

Last edited by azazello; 7th November 2014 at 07:26 AM.
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Old 7th November 2014, 08:54 AM   #5
Green77 is offline Green77  Sweden
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I messured some DC last night, but i will check more tonight.

Here is the schematic:

Click the image to open in full size.

I have checked all resistors and they seems fine and all value are ok. The only thing was R8 should be 1 Mohm but i messure 1,2 Mohm.

So if the hiss and hum goes away when pulling the P.I tubes it must be something with the PS for the "preamp" section or
with the audio path for the "preamp"?
With no P.I tubes installed but the KT77 installed and bias to 50mA each the speaker are dead sillent.

Regards //Daniel
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Old 7th November 2014, 12:37 PM   #6
DF96 is offline DF96  England
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Loud hum and hiss can be a sign of parasitic oscillation. Measure DC voltages around the phase inverter, preferably with a 1M resistor attached to the DMM probe to isolate RF. Assuming a 10M DMM you will read 10/11ths of the voltage.
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Old 7th November 2014, 04:48 PM   #7
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Try disconnect 100 uF from cathode of first pre tube.....Re-solder all solder in preamp.
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Old 7th November 2014, 04:53 PM   #8
Green77 is offline Green77  Sweden
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Weekend just started here in Sweden. Nice!
I will start messure dc soon and desolder the 100uf cap and resolder
all points.
Regards //Daniel
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Old 7th November 2014, 07:55 PM   #9
seppoa is offline seppoa  Sweden
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Im building exactly the same amplifier also, with EL34. Check without the output tubes in place, that you have sufficient negative bias at pin 5 at the sockets.
Cheers,
Seppo
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Old 7th November 2014, 08:22 PM   #10
Green77 is offline Green77  Sweden
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Hello Sweden!
Bias volt ok. Set to 50ma per tube.
It's playing now. Really nice!
But it's only working when i play from my mobilephone to rca in.
When i try from my dac i get loud hum and hiss!!! and nothing with
my dac.
Regards //Daniel
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