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Looking for some help

172.56.19.120
Posted on December 8, 2014 at 13:24:07
David Smith



Working on the above design, part way through obviously. This is my first design from scratch, and not really helping myself with some complexity but in any case I'm giving it a shot. A few things need revision (300b cathode resistors at center taps not filaments, many values left to calculate), I'm obviously not done but had questions before spending too much time in the wrong direction.

The biggest area of uncertainty for me is the driver. Does the requisite cathode voltage develop from circuit conditions or do I need to apply a voltage to the top of the CCS? Does this input/driver DC arrangement have a hope of being stable?

Any other reasons this is a non-starter?

Thanks in advance.

Dave

 

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RE: Looking for some help, posted on December 8, 2014 at 13:43:12
hifipaul
Audiophile

Posts: 735
Location: NY
Joined: December 22, 2008
On the 6j5 I'd suggest using a diode in place of the cap and resistor.

If you gotta use 6v6s, try them at lower current, and consider using a 6bx7, 6bl7, or 6ah4 in place of the 6v6s.

Look up the Mullard circuit for hints on how to reference the 2nd 6v6 grid to ground.

The current source is a good idea. It needs trimmer pots in order to modify current flow.

 

RE: Looking for some help, posted on December 8, 2014 at 15:49:06
David Smith
 Thanks Paul. Why do you advise against 6V6 or running them at 30mA? I considered 6BX7 but most comments I've read about sound are not positive. I'll have another look at the Mullard circuit, thanks.

Dave

 

RE: Looking for some help, posted on December 8, 2014 at 16:24:00
Stuben
Audiophile

Posts: 669
Location: Guber Ohio
Joined: December 30, 2005
 Hey David,

Unless you're dead set on not using solid state devices, I'd suggest an easy to build coscoded mosfet CCS kit for the tail of your divider.Just to save your sanity for more important challenges...you can also load your driver plates with a good CCS...I like the sonic results that I get over a resister...The 6BX7 is a good choice and it is already a triode...

Get some sound out of that puppy and then tune...

Stuben

 

RE: Looking for some help, posted on December 8, 2014 at 19:52:25
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015
 In the schematic, the heaters of your 6CL3 rectifiers for the plate supply are shown series connected across a 6.3V winding. They are 6.3V tubes. They would need to be parallel for 6.3V, or you could series them with a 12.6V winding.

 

RE: Looking for some help, posted on December 8, 2014 at 20:37:16
David Smith
 Thanks, stupid mistake that I just might have failed to catch.

Dave

 

RE: Looking for some help, posted on December 9, 2014 at 03:06:34
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 6V6s in triode sonics vary a lot depending upon the manufacture. I never found one I liked beyond the old Sylvania 6V6G 'coke bottle' glass and the old Sylvania 7C5 with the small oval plate (VT type mil spec tube).

IMO, less risk would be using a true triode driver. CCS in the tail of the phase splitter preforms well. The 6V6 bottom tube grid is not connected.

 

RE: Looking for some help, posted on December 9, 2014 at 17:59:04
hifipaul
Audiophile

Posts: 735
Location: NY
Joined: December 22, 2008
You are better off sonically with a true triode than a triode strapped pentode. The 6AH4 is likely your best bet. I think VTA is selling that circuit as a kit using 6sn7s, it is made to fit in a ST70 chassis.

 

RE: Looking for some help, posted on December 9, 2014 at 22:55:05
David Smith
 OK thanks, I'll look into those.

Dave

 

RE: Looking for some help, posted on December 9, 2014 at 22:59:02
David Smith
 I am a bit surprised at the negative comments about the 6v6's based on what I've read in the past, but I'll take that into consideration. Yes I should have finished drawing the driver circuit before posting, I have since fixed that but thanks.

Dave

 

RE: Looking for some help, posted on December 9, 2014 at 23:00:11
David Smith
 More food for thought, thanks.

Dave

 

RE: Surprised about 6V6 comments, posted on December 10, 2014 at 04:59:10
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 The 6V6 in triode has great plate curves- drew me into building a PPP triode connected 6V6 amp years ago. Problem was the large stash of NOS RCA metal 6V6 I had sounded awful. Finally, the 7C5 bailed me out as finding 8x 6V6 Sylvania G glass did elude me. I did borrow four Sylvania 6V6G glass for testing in one amp. (I built monoblocks).

In your case for a driver you have more options. I found the 6SN7 sounds better than a 6BX7 or 6BL7. Not all 6SN7s worked for me either. The Sylvania 6SN7GTA with the plates on an angle sounded most clear.

Granted the 6BX7 or 6BL7 has more drive capacity, but the 6SN7 provides enough drive. I would use a Tung Sol round plate 6SL7 as the preamp tube. find more magic with a high gain preamp tubes and the 6SL7 has less miller capacitance issues vs a 12AX7.

Use a coupling cap from 6SL7 to 6SN7 and place a CCS on the 6SN7 cathode. Then, you can ground that bottom grid. That means no capacitor in the grid to ground. The TT phase splitter will be very accurate & accurate phase splitters is the trick for PP amps to sound about as good as SET amps in the midrange plus PP are more dynamic without a loose coupled SET audio transformer.

200 volts plate at 14ma or 250 volts at 9ma per 6SN7 plate is in a good part of the 6SN7 plate curves.

My 2 cents worth.

 

RE: 6SL7, posted on December 10, 2014 at 05:19:54
David Smith
 Why the 6SL7 over 6J5? I think I should get enough gain with the 6J5, but perhaps I'm wrong. Is it a sound thing or you think I need the gain?

Save

 

RE: 6SL7, posted on December 10, 2014 at 06:04:52
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 The 6J5 (1/2 of a 6SN7) does have way enough gain for the application. The operating point on the plate curves is good. The right brand of 6J5 should sound excellent.

My notes over the years have shown higher gain front-ends of amps sound better with the right high gain tubes. I am not opposed to cross over to the 'sinful dark side' using a pentode preamp tube either. The stellar performing H&K Citation II and Marantz 8B that use pentode preamp tubes come to mind.

I like the Tung Sol 6SL7 round plate as a preamp tube is all. Nothing wrong with a 6J5 tube as seen on the paper specs. Being the 6J5 is 1/2 of a 6SN7 and I do not like the sound of many 6SN7s, simply choose a great sounding 6J5. Perhaps a Sylvania 6J5GT with that same plate as my favorite Sylvania 6SN7GTA exists.

Bottom line is IMO the brand of tubes make or break an amp.

Edit- I remember using a Sylvania 6J5GT with a metal base and black round plate. That is a good sounding tube!

 

RE: Looking for some help, posted on December 10, 2014 at 08:23:14
truep
Audiophile

Posts: 375
Location: Texas
Joined: March 28, 2004
 I don't see anything connected to the control grid of the bottom 6V6.

 

Thanks, posted on December 10, 2014 at 08:26:49
David Smith
 Much appreciated. FWIW the input tube of the 8b is triode-wired I think, but interesting comments about the high-gain input. I do have one of those Sylvanias.

Dave

 

Yes that's missing, the schem is incomplete, posted on December 10, 2014 at 08:28:54
David Smith
 I wanted to get comments if the general topology is workable before spending too much time on it. Sounds like it is with some useful suggestions/comments below.

Dave

 

Sylvania 'SN7, posted on December 10, 2014 at 14:23:09
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 "The Sylvania 6SN7GTA with the plates on an angle sounded most clear."

Interesting you say that, I absolutely agree. I had a chance to buy a good number of the Sylvania 12SN7 chrome tops last year at a really good price. They were marked Zenith, and that apparently put most buyers off. What I found though is that the sections in each envelope were much more closely matched than identical 12SN7s marked Sylvania. I can only assume that Sylvania screened them for this before printing the Zenith name on them. All of these are a world better than RCAs from the same era, frequently so mismatched in the same bottle that I can't use them.

 

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