Loading the outputs of a tube amp

[mjrippe]

try the 250 ohm tap and measure  thd,  would be interested in seeing if using the whole winding improves distortion and upper freq response,

The 250 was the same THD+N as the 16 

 

[buildafriend]

I don't see a 6.6k primary but I see 8k with 600 ohm out from edcor.. 10W or 25W. I'm not sure if it would even help that much but since the STA level uses push pull out and then go's down to line level with an H pad and sounds amazing it makes me think there is probably a way to do this..

https://www.edcorusa.com/cxppseries#/specFilters=9!#-!259!-#!14m!#-!199

 

[gridcurrent]

attaching  partial schematic found online.
amplifier is low power but fixed bias.
if mine, would add at least 1, maybe 2 RC sections of power supply filtering,
then convert the output tubes to triode operation self biased.
yes, maybe more work, such a nice package including the plug-in input transformers.

 

[abbey road d enfer]

113db of gain on a microphone preamp? That has to be a new record!

It may seem ridiculous, but consider it's not comparable to a standard mixer.
That's the gain from the 30 ohm input to the 70V output.
70V is +39dBu. With 113dB of gain, that amounts to an input level of -73dBu (0.17mV), corresponding to about 84dBspl for a typical period mic.

 

[abbey road d enfer]

if mine, would add at least 1, maybe 2 RC sections of power supply filtering,
then convert the output tubes to triode operation self biased.

Why would you do that?

 

[Tubetec]

Trioding the EL84's sounds like a good plan to me ,
You'll reduce the output levels to better suit other gear and maybe get some reduction in distortion compared to pentode , Maybe you could use feedback across the output transformer to clean things up a bit more
I cant seem to find a schematic for it although Radiomuseum has a few photos .
The swapable octal input transformer in conjunction with different mics gives the unit plenty of tone scope , HPF on the mic channel might be easy to add .

One of these little boxes looks like a great way to get some 50's vibe in your recordings ,
Another thing you can easily do with this kind of amp is add a couple of reverb tanks off one of the output taps via an attenuator , feed the recovered 100% wet reverb signal to another DAW or mixer/recorder input , now you have your direct signal from the pre amp and reverb in parrallel , you set the levels and your ready to record .

 

[abbey road d enfer]

Trioding the EL84's sounds like a good plan to me ,
You'll reduce the output levels to better suit other gear

Gain and output would decrease by about 6dB. It's not enough to match with most line level equipment. I have no doubt that an output attenuator or a complete redesign of the output stage should be used.

 

[Tubetec]

Hard to say much without a circuit diagram , but a better class of sheilded line level output transformer would help . Removing the power supply lump and its caps to a seperate chassis even a few feet away might get rid of a nagging low level hum ,but often you can improve on the amount of hum by balancing the cathode currents of the output tubes so noise cancels better in the output transformer primary . The Williamson arrangement is nice but requires two pots where only a single pot will balance the current but not allow you vary the overall current .

 

[Lawrocker]

I just refurbed a couple Altec 342B Mixers and put an 'L' pad on the output. This was pulling the 4 ohm tap off the back of the barrier strip and inserting a 1W 470 Ohm resistor in series there and another 470 from 4 ohm tap to common. Worked great. Made an XLR Male Pigtail and done.

 

[abbey road d enfer]

I just refurbed a couple Altec 342B Mixers and put an 'L' pad on the output. This was pulling the 4 ohm tap off the back of the barrier strip and inserting a 1W 470 Ohm resistor in series there and another 470 from 4 ohm tap to common.

That means the amp's output is loaded with 100+ times its nominal load. This is not recommanded practice, since the power amp may run into oscillation. You're lucky it works for you!