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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 30th July 2013, 02:23 AM   #1
Sgrilli AD is offline Sgrilli AD  Brazil
diyAudio Member
Join Date: Aug 2012
Location: Poços de Caldas - Minas Gerais
Default Another Simple SE lives as prototype

Hello all,

My SSE is alive, even as prototype yet, but working well. I assembled a test bench almost similar like the one that George recommends on the site and did the initial tests.

My setup is:
Allied 6K7VG
Edcor GXE 15-8-5K
Triad C-14X (6H; Tol -20%; +50%; 200mA; DCR 150 Ohms)
ECC81/12AT7 NOS Germany (RFT / FUNKWERK)
6P3S / 6L6GC NOS FOTON PlantTube (former URSS) 1960s
0,22uF 500V PIO K42Y-2 NOS Russian
680 Ohm cathode resistor for 6L6 tubes

I ordered a NOS RARE SVETLANA =C= Plant (former URSS) 5C3S tube NOS 1980S, but I made a msystake when I order the C1 caps (47uF) so I can not use this tube for now.

Measurement results:
With 6L6Tube, 680 Ohm cathode resistor, SS rectifier, triode mode and no CFB. This is the simplest way in the wiring diagrams, see figure at this link: http://www.tubelab.com/images/Assemb...rounding_1.jpg

As I can not use the 5U4G tube, I left empty the V2 and put a jumper on the SW1 in the PCB. Things went well, but I do not know if this is the best way to do. Without the jumper I had no voltage on pins 3 and 8.

Tubes V11 and V21 (6L6)
Plate (pin 3) = 491 e 495 VDC (two measures)
Cathode (pin 8) = 40,5 VDC
Heater (pin 2) = 33,1 VDC
Screen (pin 4) = 495 VDC
R17 e R27 = 662 OHM and 40,6 VDC

Tube V10 (12AT7)
Plate (pin 1) = 254 VDC
Cathode (pin 3) 2,16 VDC

Well, I think the plate voltages in V11 and V21 are quite high. I read that the working voltage of the 6L6 is around 250V (GE datasheet) I do not have the datasheet of my tubes.
But, things seem fine. No smoke, no smell, transformer are so cold, no component burned, even fuse. The sound is good, although it is still early testing.

I ordered a pair of EH KT88 but has not yet received them. I plan to buy EL34 also and run with a cathode resistors switch.

I'd like to hear from you if I'm on the right way.

Thank you and sorry for the too long thread.
Sgrilli AD

Tuesday - 12:53 p.m.
I think I found the reason for the high voltages of the plate.
The power transformers Allied 6K7VG is rated as 117VAC on the primary, but the grid here in Brazil can reach 127V. Right now I measured 125.9V

Before I starded to assembling the SSE, I measured 124V and sent E-mail to the technical staff of Allied. The answer was as follows: "The transformer has a nominal input of 117V but 124V won’t hurt it. It will however increase the output voltage by the same percentage. 124/117 = (1.06 * the output nominal voltages)"

So, using the same logic, today I got 125.9/117 = 1.076
Allied has 750VCT on secondary (red wires): 750 * 1,076 = 807VCT
I measured 840V (425 - 415) in the "T1-Red" terminal on the PCB. I think that is consistent.

I´ll try to put more resistor in parallel with R17;R27.

Are there other ways to reduce the plate with this input values of Allied?

Many thanks

Last edited by Sgrilli AD; 30th July 2013 at 04:26 PM. Reason: more information available
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Old 2nd August 2013, 09:32 AM   #2
rmyauck is offline rmyauck  Canada
diyAudio Member
Join Date: Apr 2009

Resistors hurt the sonics by increasing the DCR in the PS hurting the dynamics of the sound. A better way might be with a bucking transformer of say 6-12 volts with enough amperage to work. If you do a search you will find info on the forms how to wire it correctly.

Some people also run there amp with a variac (5A +) adjusting for the changing line voltage.
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Old 2nd August 2013, 05:53 PM   #3
Sgrilli AD is offline Sgrilli AD  Brazil
diyAudio Member
Join Date: Aug 2012
Location: Poços de Caldas - Minas Gerais
Understood rmyauck.

I think the variac is the best option.

Just another question:
In the case of my measurements, which is really my B+?
  • Plate (pin 3) = 491VDC
  • Cathode (pin 8) = 40,5VDC
  • Heater (pin 2) = 33,1VDC
  • Screen (pin 4) = 495VDC
I read a lot about it and I'm a little confused by the information I got. Some guys says that B+ is the value measured at pin 3 (plate) to ground and others say that is the value measured at pin 3 (plate) to pin 8 (cathode).
And there are others who say it is a result of plate voltage minus cathode voltage. If this is true, then my B+ would be OK, because it would be 491 - 40,5 = 450,5VDC

Thank you.
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Old 8th August 2013, 11:30 AM   #4
Sgrilli AD is offline Sgrilli AD  Brazil
diyAudio Member
Join Date: Aug 2012
Location: Poços de Caldas - Minas Gerais
Default B+ Voltage

Someone else could help?

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Old 8th August 2013, 12:17 PM   #5
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
The operating point of the tube is the plate (anode) voltage minus the cathode voltage, and the anode current, which in a pentode is dominated by the grid to cathode voltage and screen voltage. Check in a dark room if in any condition of working (full or no power) none of the internal electrodes of the final stage becomes red (Too hot). If this is not the case, a little distortion can be present if a mistake in biasing the tube, but if no red, the valve is in its SOA.

Usually a Class A amplifier has it maximum plate power when no signal and is no giving power to the. load.
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.

Last edited by Osvaldo de Banfield; 8th August 2013 at 12:22 PM.
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