6V6 SE VS ECL85

[J_Hart_14]

So, I have build a couple of SE stereo amplifiers using the ECL85. I will include the schematic for what I have currently. What I want to do is essentially use all the same transformers, choke, and power supply and substitute 6v6 for the pentode and a 12AT7-WA (I just have a bunch of these laying around) for the triode gain stage. I wanted to see if anyone had some experience running the voltage that low on the 6V6 and my power transformer is going to limit the current available for the power stage to about 30ma per channel (worked out just fine for the ECL85 but I think it is not going to be enough for the 6v6). Thoughts?

[artosalo]

I wanted to see if anyone had some experience running the voltage that low on the 6V6

I think your supply voltage is no wise low. Seems to be over 400 V.
Since the available current is limited, the output power will stay at the same level ( about 3 W ?) as with ECL85.

There seem to be absolutely too small cathode bypass capacitors at the pentode, only 6 uF. This will limit the bass response. Put min. 100 uF instead and you hear the difference.

[azazello]

You need for heating 6V6 and 1/2 12At7 about 0.6-0.7 A per channel.
And 50 ma +B per channel, 6V6 works good with 40-45 ma - 280-300v., and 12AT7
with about 5-7 ma-180-200v.
Suitable transformer for PSU is Hammond 269AX - I built the same ampl. few days ago.

[J_Hart_14]

I think your supply voltage is no wise low. Seems to be over 400 V.

Just to clarify that the supply voltage ends up being around 220-230v the transformer and the type of rectification really makes the transformer a 175-0-175.

There seem to be absolutely too small cathode bypass capacitors at the pentode, only 6 uF. This will limit the bass response. Put min. 100 uF instead and you hear the difference.

Also to address the capacitor size I ended up putting a 47uF cap since that is what I had in my parts bin. If I remember right on the schematic the reason I said min of 6 uf was for a 3db roll off that gives about 40Hz. I agree low end would suffer. 47 uf would give roll of of about 5Hz or so. Maybe I am missing something with this calc though?

6V6 works good with 40-45 ma - 280-300v

I only get about 220v for the high voltage out of my power supply. 40-45ma x2 channels (80-90 ma) will overload the transformer.

Have very many people run a 6v6 at 200v (with the type of bias setup I have here) and about 30ma with success having a 6.6k ohm primary load to 13k primary ohm load?

[azazello]

Hammond 269AX has 250 v - 100 ma on secondary coil. /cost about 39 $/

[J_Hart_14]

Hammond 269AX has 250 v - 100 ma on secondary coil. /cost about 39 $/

I think that transformer would work well I was just trying to avoid buying another power transformer and using up the parts that I had on hand. I might have another power transformer that would give me the voltages and current capability that I am looking for. Maybe the best thing is to get a little bigger transformer?

[tubelab.com]

I use the Allied 6K56VG in my 6V6 Simple SE. It uses a 12AT7 for the driver. B+ is 325 volts using a 5AR4 rectifier tube. Probably around 350 volts with silicon. The transformer is $45 and is made by Hammond.

Search results for "6k56vg" - Allied Electronics

[Printer2]

Or if you have another transformer that puts out 12v you could free up some of the iron in your existing transformer and not run the heaters off it. That may give you an added 25mA which should put you in the ballpark for the 6v6's. If you find you like it you can look at a permanent solution.

[J_Hart_14]

Well, after going through thanksgiving and working through the design. I had a spare transformer that was a little larger that could handle the voltages a little better. Here is a schematic of what I ended up going with. I am still up for some modifications if someone has any. Also I have one correction to the schematic that I have drawn. I have a choke from the diodes to the first capacitor. The choke is 7H and 240ohm. Let me know what you think. I have also included a picture of the amp. Thanks in advance.

[artosalo]

I have a choke from the diodes to the first capacitor.

If you still look for a bit higher anode voltage, put an electrolytic capacitor before the choke. This could rise the supply voltage a little.

Couple of other comments;

You could use separate cathode resistors at output tubes. This improves possible unbalance between tubes.

Since your output transformers are not the best for 8 ohms load ( too high turn ratio) you could also add a light NFB, for example by connecting the secondaries of the output pubes in series with the cathode resistors, (and with correct polarity).

Usually the linearity of the output stage is improved if some 470-680 ohms series resistor added between UL-tap and g2.