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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Simple Hi-end 845 amplifier

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#41
No, in this case i haven't reduced the Ra or Rk on the 1st valve. I want to be able to put the ecc83 back there at the end of the day.

I learned this trick from my guitar playing days. 😉

I have tried a few different tubes. Philips, telefunken, rdf, tesla, etc. The mullard cv4004 and cv4003 are the best sounding ones I have but they are are not cheap or easy to find.

The e88cc has a nice flat response and but not as much gain as the ecc83, and I don't think it can take so many Volts to the plate...

I was thinking along the lines of JRB's "Improved DC-cascaded feedback pair" for the gain stage. This would replace the single ecc83 and add a coupling cap - but it would result in a flatter response curve

img20.gif


The value for Ra is 10k (ie 20k on the first valve, 10k on the second). Rk for the first valve would be 400 Ohm, and RK for the second would be 10k. Rfb is also 10k. You don't need the diode if you have a slow start.

I like this circuit, but it calls for a B+ of 300V. I would have to re-work the power supply... and elevate the heater voltages to correct levels (70 and 170 volts)...

What do you think?
 
#42
soulmerchant said:
No, in this case i haven't reduced the Ra or Rk on the 1st valve. I want to be able to put the ecc83 back there at the end of the day.

I learned this trick from my guitar playing days. 😉

I have tried a few different tubes. Philips, telefunken, rdf, tesla, etc. The mullard cv4004 and cv4003 are the best sounding ones I have but they are are not cheap or easy to find.

The e88cc has a nice flat response and but not as much gain as the ecc83, and I don't think it can take so many Volts to the plate...

I was thinking along the lines of JRB's "Improved DC-cascaded feedback pair" for the gain stage. This would replace the single ecc83 and add a coupling cap - but it would result in a flatter response curve

img20.gif


The value for Ra is 10k (ie 20k on the first valve, 10k on the second). Rk for the first valve would be 400 Ohm, and RK for the second would be 10k. Rfb is also 10k. You don't need the diode if you have a slow start.

I like this circuit, but it calls for a B+ of 300V. I would have to re-work the power supply... and elevate the heater voltages to correct levels (70 and 170 volts)...

What do you think?
Click to expand...

My objection to this circuit is that it uses global feedback to control the overall gain, which also impacts the distortion spectra. Depending on tube type chosen the open loop gain can be rather large resulting in a high feedback ratio which in turn can give rise to a very extended un triode like distortion spectra. (Admittedly this is low level, but this is one of the many complaints about high feedback ratio SS amplifier designs.)

JMTCW...
 
#44
Well then how about this? 😀

img28.gif


"Compound amplifier with constant current sources and DC coupling between stages"

I have built one of these as a single gain stage after my RIAA phono amplifier and it is really superb. You will need to incorporate some SS in the design though...
 
#45
Regarding the el34 PSE - changing Rk to 2x240k (= 120k) and leaving Ra 2x4.7k for ecc82 instead of ecc83 as driver (?)

But I am still thinking about another gain stage and driver.

The e88cc with its maximum plate value of 220 V obviously won't work as driver... I think a 5687 could work though. It has a max plate value of 500V and a plate resistance of only 2.75k (less than with a ecc82).
 
#46
soulmerchant said:
Regarding the el34 PSE - changing Rk to 2x240k (= 120k) and leaving Ra 2x4.7k for ecc82 instead of ecc83 as driver (?)
Click to expand...

Please ignore this - it is definitely not right. I hope you can be patient with me...

for Vg of -2.0 and a Va of 170V there is an Ia of about 16mA from a Philips ecc82 standard curve...

Ra = (SupplyV - AnodeV) / Ia (ohm's law)

Ra = (430V - 170V)/16mA = 16.3k

and

Rk = 170V/16mA = 10.6k

Problem here is it is a low Vg for ecc82 (I think - please correct me if I am wrong).
 
#48
 

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#49
I see quite a lot of 100Hz in the spectra and in the mls response as well. Any idea where that is coming from? It's only 60dB down from the fundamental and would be quite audible I suspect.

Judging from the drop in LF response with the 2A3 the plate resistance is significantly higher than in the EL34 which actually doesn't make much sense, they should be within about 100 ohms of each other unless the 2A3 is running at a fairly low plate current. High frequency behavior was a little odd as well. Did you use fixed bias with the 2A3?
 
#50
The 100Hz are generater by the ac filaments used in this test.

In any case I was surprise by this bad result with 2A3.
agon EL34

I have used 2A3C Shuguang mono plate and EL34WXT Sovtex or EL34 Premiun Golden Dragon (same result).

Have you other tube choices ?
 
#51
I've never been terribly impressed with the Shuguang 2A3 not to mention some of their other products, but I have heard that their QC has improved considerably since the last time I tried them.

An inexpensive 2A3 (also mono plate) from Sovtek might be worth evaluating. I think you might also want to try either a filament balance pot on the 2A3 filaments or dc filament power.

I don't know what plate current you are running those 2A3 at, but I would think something around 50mA would be about right. (Run them just below what the interstage transformer is designed to handle taking into consideration what your power supply can safely run at.)
 
#53
audiodesign said:
For the NC20 the max current is 30mA and this is the max value tested for the 2A3.

So I think that to use the 2A3 like driver it is necessary have an interstage transf. with 100H, but there is no product on the market with this characteristic
Click to expand...

The Rp of the 2A3 as compared to the EL34 is pretty close, I shouldn't think anything over 25H should be necessary provided that the 2A3 is run at the recommended current of about 60mA..
 
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