Info for 6b4g PP

[sparkx]

Hello. This is my first post.
I have four 6b4g Svetlana and I want to build a PP amp.
I need schematics with:
- good sound;
- simple to build;
- not expensive (no interstage transformers);

For example, is this scheme good?
JE Labs PP 2A3 Amp Project

Best regards,

sparkx

[AndreasS]

Hello Sparkx,

a cheap amplifier schematic you ca find there:

6B4G-Gegentaktverstaerker mit Hammond-Transformatoren, von Andreas Schwarz

The interstage Hammond 124E is connected as a (differential) plate choke (increasing the output swing of the 6SN7 driver).

Regards Andreas

[SY]

Originally Posted by sparkx

Hello. This is my first post.
I have four 6b4g Svetlana and I want to build a PP amp.
I need schematics with:
- good sound;
- simple to build;
- not expensive (no interstage transformers);

For example, is this scheme good?
JE Labs PP 2A3 Amp Project

Best regards,

sparkx

Welcome to the forum!

Let me start by pointing out the foreign object floating in the punchbowl- a decent DHT tube amp will NOT be cheap, even without an interstage. The schematic you linked to is a very unimpressive design (assuming you want an amp whose output replicates the input as opposed to an effects box)- it's basically a Mullard 5-20 with the feedback removed and a lower gain tube used in the first slot. The phase splitter stage is unbalanced, and the HF bandwidth is limited. The distortion will be high.

You can do better.

edit: I noticed that the writer began the linked article by saying that he really doesn't know how to design amplifiers. When someone tells you that, it's best to believe him.

[mr ed]

I built a single ended amp using two of these tubes. I works quite well.
You can build from this link below and its pretty easy.
diytube.com :: View topic - get*set*go docs & mods *4/6/09*

Ed

[Eli Duttman]

Sparkx,

How well matched are your 6B4G pairs? The J.E. Labs design is a Mullard circuit sans GNFB loop. It's easy enough to change from the self bias shown to "fixed" bias, which will deal with cathode current differences. However, unless the gm matching is reasonably close, you will have issues.

UF5408s in the PSU will be much quieter than the 1N5408s shown. Replacing the 12 KOhm LTP tail resistor with a 10M45S CCS is a worthwhile improvement. A separate 6.3 VAC winding and 50 Ω balance pot., for each 6B4 filament, will allow good control over hum.

Have you given thought to the O/P "iron".

[Yvesm]

Originally Posted by SY

The phase splitter stage is unbalanced, and the HF bandwidth is limited. The distortion will be high.

Hi Stuart !

Had an hard night ?

Trash all your tube data sheet as well as all your measuring gear and you'll no longer have problem about distortion, bandwith and all this kind of details

Try more coffe !

Yves.

[sparkx]

Ok. Thanks for replies!

Originally Posted by Eli Duttman

How well matched are your 6B4G pairs?

The four 6b4g are matched by Billington.
I have a pair of 5k PP output transformers C-core of good quality.

Originally Posted by SY

and the HF bandwidth is limited. The distortion will be high.

Why?

Best regards,

sparkx

[SY]

Originally Posted by Yvesm

Hi Stuart !

Had an hard night ?

Yeah, Rhone wine. You Frenchies are diabolical!

Sparkx, HF bandwidth will be limited by two sets of significant Miller rolloffs. Distortion will be high because of the unbalanced drive from the phase splitter, the 120Vpp (or so) that the phase splitter will have to swing, and the reactive load of the phase splitter on the input stage. Again, some might like these effects (it's like a treble control turned down and an Aphex box turned up), and I can't argue that- thus my caveat about the amp as a pure gain device versus a device meant to change the sound.

For a really detailed analysis of the 5-20 input stage as well as detailed examination of the long-tail pair phase splitter, I'd highly recommend "Valve Amplifier," 3rd edition. It will cost about the same as a pair of 6550s and provide an excellent background for you to work from.

[SY]

Looking again at the HF rolloffs, they're higher than my first glance suggested. So strike that criticism. I'll stand by the high distortion and high output impedance criticisms.

[Eli Duttman]

Sparkx,

Let's deal with SY's legitimate objections, in a cost effective manner.

The 1st C_Miller induced low pass pole is at the circuit's I/P. The grid to ground resistance of 270 KOhms is too large. Reduce that value to 100 KOhms. Also, the 6SN7 is not highly prone to parasitic oscillation. The 10 KOhm value for the grid stopper is complete overkill. Make that part 510 Ω/Carbon composition.

I previously stated that installing a CCS, instead of a resistor, in the LTP's tail improves performance. A CCS in that position forces symmetry between the 2 halves.

The 2nd C_Miller induced low pass pole is at the 2A3 grids. Once again, the 270 KOhm grid to ground resistance is too big. In addition, switching to "fixed" bias (for balance reasons) requires the net grid to ground resistance to be quite low. Otherwise, the DHT can run away. Good as it is, the 6SN7 is not up to the challenge of simultaneously working into a low net impedance, driving the 2A3's considerable C_Miller, and swinging 120 or so V. peak to peak. A change in type is definitely in order, for the LTP. I suggest the ECC99. While the μ of the ECC99 and the 6SN7 are roughly =, the '99 exhibits both a higher gm and lower R_P. Those electrical characteristics are very important, when driving 2A3s.

BTW, in his write up, J.E. mentioned a high/low PSU. That's a direction this project will have to move towards.