Question about DIY EL34PP amp

[vangogh-hk]

1 am going to build a EL34 PP amp using the circuit below

Page 19,20 of
http://www.lundahl.se/pdfs/claus_byr...fier_30wpp.pdf

I have got a power transformer with following taps:
180v-170v-0v-170v-180v 300mA
3.15v-0v-3.15 5A
3.15v-0v-3.15 3A
0-50v 100mA

Due to direct couple from input stage(Using ef86) the cathode voltage in the driving+phase inverting stage(ECC83) may be quite high. Is it necessary to rise the potential of heater supply line of(ECC83) to avoide flash in cathod?

The 3.15v-0v-3.15v 5A heater is reserved for EL34 as I know that it is desirable to have separate volatge supply for power tube heater(high current) and line/driving stage heater(low current). If I raise the potential of the remaining 3.15v-0v-3.15v 3A heater for ECC83 and EF86 to a level close to cathod voltage of ECC83, the potential of heater line may be too high compared with the cathod of EF86.

Can anyone suggest any solution for this situation.

Thanks

[leadbelly]

Not sure I understand the problem. The schematic shows that the design uses just one 6.3V winding centered around ground for all of the tubes, so you have a working solution by just referencing both your LV windings to ground. Do you think this is a latent defect in the design? Have you changed something in the design you haven't mentioned? Or do you just want to improve upon the design?

[cogsncogs]

vangogh-hk

In the Mullard circuit(s) the cathode/heater voltage is within the rating for the ECC83. But if you still want to bias up the filament voltage and stay away from the maximum for the EF86 of 100V, +40 to +50V (even 20V may be beter!) would be just about ideal and as an added benefit you would be biasing off the cathode/heater diode of the EF86 and may lower the heater/cathode hum of the EF86.

Cheers
Wayne

[vangogh-hk]

Thanks Wayne,

I have another question that if I am going to use this EL34 PP amp with my Ground Grid (GG) pre amp, did I need to modified the EF86 input stage circuit.

Thanks

[cogsncogs]

I have another question that if I am going to use this EL34 PP amp with my Ground Grid (GG) pre amp, did I need to modified the EF86 input stage circuit.

Depending on the gain of your GG pre-amp and the output level of your input sources; as the gain of the Mullard 5-10 and 5-20 is pretty high to begin with, you could strap the EF86 as a triode. As a result it will lower the input noise (partition noise) and lower the output impedance and distortion of the EF86. Downside: It will increase the miller capacitance, but it won't be of much concern IMO. Some people highly prefer a triode over a pentode as the first stage of amplification.

Cheers
Wayne

[vangogh-hk]

Thanks,

I am already using the scheme in page19 (The 4-30)with triod connected EF86 without cathode bypass cap. If Specification of my DIY GG Pre amp which is going to be connected to this power amp is:

Gain: 12 dB.
Max Signal Out: 20 volts RMS.
Output Impedance: 200 ohms.
Input Impedance: 50 k Ohms

Should I modified the EF86 stage or the Ecc83 stage (e.g. change to 12AT7) or just let it be.

Thanks

[Eli Duttman]

Should I modified the EF86 stage or the Ecc83 stage (e.g. change to 12AT7) or just let it be.

IMO, changing from the 12AX7/ECC83 to the 12AT7/ECC81 in the phase splitter position makes LOTS of sense. The net harmonic distortion spectrum of PP "finals" combined with a 'T7 driver is a PLEASING 2nd>3rd>4th, etc. Also, the 'T7 has a high gm. That makes it superior to the low gm 'X7 as a LTP splitter/driver. High gm types are LESS likely to slew limit. Always use grid stoppers with high gm types.

FWIW, I'd try a triode wired EF86 in the voltage gain position, as your line stage has appreciable gain. If it turns out you need some more gain, switching from the EF86 to a section of a 5965 is an option.

[vangogh-hk]

Thanks

If I replace the ECC83 with 12AT7, do I need to adjust the value of other components in the circuit?

Thanks again

[Eli Duttman]

If I replace the ECC83 with 12AT7, do I need to adjust the value of other components in the circuit?

Probably. A good "spot" for the 'T7 is 200-220 V. on the plate and Ib = 3 mA.

[vangogh-hk]

Eli

Could you tell me what value of resistor should be used on the plate (instead of 150k + 47k VR)and cathode (instead of 82K) of the 12AT7 then?

Thanks