Personal comparisons of valves suggested for DRD

[Paul Barker]

Jack in his first article on 300b DRD suggests 6n1p similarities found to E88cc parallel or 6aq8 similarites to ECc85 I’m planning D3a.

Anyway ECc85 and Russian version both have too high ri of 9k7, even parallel they are a poor choice because of the inductive load affecting the gain. But they are the outright winner on low input capacitance. If it were only one parameter that matters ecc85 is a star. The e88 cc and 6n1p are close in ri even after paralleling, and no worse than D3a ref. Capacitance. So a parallel e88cc of which I have a ton is viable.

But because gain matters when you are limited in anode volts (the nature of direct coupling) gain matters dominantly.

So I’m still with the D3a on the matter. At 57 mu the ecc85 is nearest to 77 but the stumbling block is almost 5k ri parallel.

Its clear D3 a is winner. But then the sound nature of the input valve driver contributes more to sound than the output valve according to many over the years.

I’m just going to have to hope it sounds great. I’ve never heard it in this position at home. Though I couldnt fault an amp Mark used it up front of. So I guess its a good gamble.

BUT the EL38 se I made sounds great and uses e88cc. I also know Morgan likes it.

I’ll just have to wait and see if all the versions output valves utilising D3a going forward are lacking sound quality because.

Another present obstacle is out of the 20 odd hammond 56C’s I have only found two so far. Ho hum.

First amp for DRD is 6em7 so 1 will be ok to learn from.

[Paul Barker]

http://vinylsavor.blogspot.com/2012/12/ ... h-d3a.html

Well I’ve found a few fans, and one of them uses it to drive a el34, the rest are all phono stage most 600 ohm lcr fans. But one just a normal phono stage.

So I’ll press on with D3a.

[RhythMick]

Its clear D3 a is winner. But then the sound nature of the input valve driver contributes more to sound than the output valve according to many over the years.

I’m just going to have to hope it sounds great. I’ve never heard it in this position at home. Though I couldnt fault an amp Mark used it up front of. So I guess its a good gamble.

Watching with interest ...

[Paul Barker]

I’m now looking at



But d3a hammond choke 8 mA EL84 ae interstage transformer loading the EL84 pentode cf to 300b output stage @ 5k load and opt secondary feedback. 9.3 watts 2.1 % 2nd h. But don’t forget the cathode feedback is unknown. But in my el38 amp it shed a lot of distortion I hadn’t realised was there without the cathode feedback. So the 2.1% shall be reduced for sure.

[Paul Barker]

The 300b loading verses distortion data I came up with

2k5 13.3 W 4.95% 2nd
3k5 12.85 W 3.18%
5k 9.36 W 1.85%

I’ll go option 3.

My limit for power is the use of Black Gates. But I’m saving my 1,500 vdc poly caps for the GM70.

[RhythMick]

This PDF has a table of operating conditions which I found really useful. I also like to run the WE300B with 5k primary impedance.

https://www.worldradiohistory.com/Archi ... -pages.pdf

300B OP.jpg (150.09 KiB) Viewed 32544 times

[Paul Barker]

Thank you.

My circuit for the MU3 schematic Sukuma style.

[RhythMick]

Thank you.

My circuit for the MU3 schematic Sukuma style.

Is that battery bias on the D3a Paul ? What are you using ?

Also what are the chokes ?

[Paul Barker]

The first choke is hammond 150 h 8 milliamp 156C.

Second is out of two Quad II’s in the quad schematic it says 17mA or something like that. The beauty is the dcr is all in the choke so I don’t need a big value electrolytic cap for the additional resister. Hence I am selecting for the second choke internal resistance that suits the current flow.

I could actually use my AE interstage transformers for the second choke but in all honesty I dont believe .1 uF teflon pio coupling cap is a significant loss of quality sound.i think the direct coupling era was misguided, youre using up precious voltage to avoid a harmless cap.

[Paul Barker]

I found grid a single choke is better sounding than a resistor. Especially if the resister has to be 50k. Then coupling cap geBut two grid chokes in series isn’t significant enough to waste another part on.ts karger and you get blocking distortion. Grid chokes fix issues.

A cheap interstage needs bypassing with a cap, not saying ae fit in that camp. But there is in any case no such issue with LcL youre only issue there is LF, but thats no different from a transformer, and the grid chokes isnt an issue at all, the first L may be but no more than a transformer could be.

So LcL is good and sounds good.

One Quad choke is 60h the other is 40 with similar difference in dcr. So I’ll raid other Quad II’s until I get matched pairs. Doesnt have to go 60, two 40’s would suffice but if there is another 60, all good.

[Paul Barker]

Thank you.

My circuit for the MU3 schematic Sukuma style.

Is that battery bias on the D3a Paul ? What are you using ?

Also what are the chokes ?

yes tested for voltage on charge 1,5 exactly.

Sorry I had to move the 1 meg resister

https://freeimage.host/i/qVN4g14

after re-reading the discussion of the person who tested the sound of the Mu 3 pre-amp, which was the only pre-amp he heard which didn’t alter the sound or create listener fatigue.
pentode mode EL84 transconductance 11.3 ma/V output impedance 88 ohm. HF linear to 500khz. If you have the resister on the cathode and instead of using the 4700 bypass cap you use a ac cap of 100uf you roll off bass at -1db 80hz. I am still preferring a bias that is correct with the choke dcr, than adding a resistor and bypass cap.

Instead of the Quad II choke use a phase splitting interstage tolerant of 17mA dc and drive a push pull output stage. Quad ii or not. My Schumaker interstages tolerate 20mA and amplify the output such that the ac in is same at each phase out, no step down. So I could use a pp 300b output stage for example. My best pp OPT’s are Sowter 8k.