12BH7 Self Split PP using a Merit A2901 for OT

[frankenxtein]

What I have:
Merit P-3047 PT 240-0-240v 50ma
Merit A-2901 (see attachment)
Would like to build something along the lines of Robs Deluxe Micro design w/ self split using a 12BH7A instead of the 12AU7 (6X4 rectifier , 12AT7 or 12AX7 as preamp tube)
I realize I have less than ideal OT specs to do this.
How to wire the OT I have to do this ...... say ....  Using 8ohm speaker ....

In addition I noticed this when searching for a suitable PP OT for a 12BH7 ?
http://www.mojotone.com/amp-parts/Transformers_1/Mojo-Clone-of-Wright-1580-10K4-8-16-12BH7-12AU7-PP-Output

[PRR]

> less than ideal OT specs

So what are your "ideal" specs?

Working far beyond what you are telling us, I think you call lead 1 "0" then lead 2 is 2 Ohms, 3 is 4 Ohms, 4 is 8 Ohms, 5 is 16 Ohms, and you even have 32r at lead 6. You can drive any speaker system the cat drags in.

[frankenxtein]

The original design ideal specs being a higher primary impedance 22.5K (Hammond 125A) http://www.hammondmfg.com/pdf/5c0054.pdf

[PRR]

> using a 12BH7A instead of the 12AU7

The MojoTone link you gave suggests a lower impedance for 12BH7 than 12AU7.

All of this depends on the actual voltage you get, and your hoped-for power output.

Sorry if this is not helpful.

[frankenxtein]

No you've been very helpful. Thank you , you've pointed me in the right direction.
 I'll have a look at the voltages , low power output is the goal , 1 or 2 watts. I've got a few small amps all SE , just wanted to see what this does. I really need to build a nice testing setup.
          Unlike a previous build I started here and went to something else , this one I have everything lined up. Waiting for the mailman for a couple components.

[HotBluePlates]

The power transformer indicated on the layout seems ultra-massive-overkill for a 1-2w amp.  It can deliver 315-0-315v or 275-0-275v @ 100mA, which is very much more than you'll pull.  Expect the supply voltage to be very high (and it seems Rob ran into this issue).  The problem is exacerbated by the fact the layout says 275-0-275v, but shows the red wires being used which would give the 315-0-315v output.

Very roughly,
√(2w * 14kΩ) = 236v peak
236v / 0.6 = ~400v, but
√(1w * 14kΩ) = 118v peak, and
118v / 0.6 = 200v

You could use anything between 200-400vdc as your B+ and get something between 1-2w (not much change in ear-volume). 

The mention of push-pull 6L6's on your sheet does make me wonder if your OT is "too good" (meaning it can handle very much more power than you'll ask of it).  It'll still work fine though.

[frankenxtein]

      I built a SE version (similar not quite to spec)  of the Deluxe Micro , out of a Voice of Music 733 R-R and the Voltages are wow .... really had to tweak it down. 
     Building with parts found in the bargain bin (eBay) is an education. I'm doing my part in stamping out Wow and Flutter.  All the Reel to Reels should find there way to Memphis to be re-purposed. This will be build #5.  The 1st one with parts that didn't come from a tape recorder. Not sure about the PT it may have been in a tape recorder but .....
             The Merit A2901 is NOS and I don't want to ruin it , hence my asking for advice. This is an expensive hobby even on the cheap end of the scale. Thank you for the help.   

[PRR]

> wonder if your OT is "too good"

Rated 8 Watts. Dunno why you would use 6L6 at that level; 6V6 was often cheaper. Whatever.

[frankenxtein]

Limited by the power supply on hand. Lack of experience leads me to stick to specs on what I'm doing. Low wattage amplifiers for recording are a little easier on the wallet. Well the time you add this stuff up , a person could buy a small $150 SE but where's the fun in that? I really like the preamp in the Micro Deluxe.

[PRR]

> Limited by the power supply on hand.

Actually: by choice of single 12BH7. The PT will deliver 17 Watts raw DC. Say 12W after the bottle rectifier. You want 10W-12W of power tube dissipation. One 12BH7 is only 7W. Two parallel would be able to absorb "all" the power the PT can supply.

But that means another socket, and re-figuring the loading, and 1.5X-2X power output is not a lot louder, and may be more than you want. And it is not bad to under-load a PT, it will live longer.

[frankenxtein]

My hearing is already pretty bad. 

Some of these PT are probably under rated , even with the Bucking transformer dropping our 122.5vac to 116vac they put out pretty good , even under load.

Durable is good , wouldn't want to mess up these new antiques. Most of these PT/OT have been on the planet longer than me and will be here longer yet.  Got a Gemini Cable TV box and a grinder. The rat I hired to gnaw square holes has a dental appointment.

[frankenxtein]

Got it set to get both 4.46 ohms 1 & 3 and 8.65 ohms 1 & 4
 
Have to get some washers for the pots , using existing holes , filling holes in these boxes is fun ..... yeah it's not pretty. It'll be sturdy. Back to the hardware store tomorrow.
          (Just discovered the 22ga 600v pvc wire I bought last month is 300v. I don't want to say where I bought it here in Memphis. It's way better wire than you'll find anywhere , inside a Mayfair FT-501 Reel to Reel Japan 1962 but ,....)

[frankenxtein]

I should note this is not the same amp as Rob built due to the many factors. Lower voltage power transformer , different tubes altogether and , no bypass caps on V1.

 Well it worked from the first turn of the switch but was pretty dull.... blah. So .....
It could probably use even more tweaking , substituted/removed a few parts. It's quiet , not loud but , not a lot of bottom end. I'm used to biasing single ended power tubes so ... I wonder if I should increase the V2 cathode bias resistor more?
I appreciate your input 

PT no tubes 507vac , 501vac with 6X4 installed
B+ is 350vdc with just 6X4 tube all other measurements with all tubes installed. All plate voltages are from cathode to plate.

B+1  289v
V2 12BH7A (RCA)
plates 275v  drop at cathode resistor is 12.15v using @676 ohm resistor
(No cathode bypass capacitor)
( 12.16 divided by 676 = 0.0179 X 275 = 4.92 )

B+2 264v
10k dropping resistor
V1 12AT7 (Westinghouse)
p1- 140v
p3- 1.8v
p6- 137v
p8- 1.8v
I took off the bypass caps to get rid of squeal when volume control was maxed and also reduce gain.

[frankenxtein]

Well Kind of ruined my tests by switching the OT to the 8 ohm output.  Was running off the 4 ohm tap.    It's a little cleaner and louder. It seems to bog down when maxed out. It's running cooler than it did when I first started with the spec parts. PT was warm and the 12BH7 was a finger burner. The more I read the 12BH7 is pretty good with the OT 14000 primary and 8 ohm secondary.

Changed the cathode resistor to 818 ohms (parallel 1000 + 4700 = 824.56 ohms)

V2 12BH7A (RCA)
plates 283v  drop is 13.v using @818 ohm resistor
Dissipation is lower
( 13.0v divided by 818 = 0.0157 X 283 = 4.49 )

Preamp changed slightly

V1 12AT7 (Westinghouse)
p1- 144v
p3- 1.85v
p6- 140v
p8- 1.87v

Should I stop or continue ? 

Since no one is stopping me lol 

V2 12BH7A (RCA)
plates 286.7v  drop is 13.9v using @1074 ohm resistor
Dissipation is lower yet
( 13.91v divided by 1074 = 0.01295 X 286 = 3.7123 )

V1 12AT7 (Westinghouse)
p1- 152.7v
p3- 1.975v
p6- 149.3v
p8- 2.0v


Well lowering the B+2 drop resistor to 4700 ohms raised the voltage slightly at V1 upping the cathode resistor to 1074 ohms on V2 raised the voltage 287.6volts on the plates and 13.9 on the resistor. Added spec cathode bypass cap 25uf , just made it fart more.

For S's & G's ...... I changed tubes to some different tubes
V2 12AU7 (Baldwin NOS)
plates 287.1v  drop is 11.89v using @818 ohm resistor

( 11.89v divided by 818 = 0.01453 X 287.1 = 4.173 )
V1 12AT7 (JJ New)
p1- 136.8v
p3- 1.99v
p6- 133.1v
p8- 2.0v

In conclusion the PT w/6X4 Rectifier is too under spec but , being this (non spec) amp is so quiet it would be great for recording or an apartment.  Plenty of distortion from V2 with either power tube.  The bypass cap on V2 still makes it fart and not enough clean sound. I'll have to revisit the other Micro Deluxe and maybe swap some parts here and there. Use the 12BH7 and the Merit A2901 in it. Plenty of power from the VM 733 PT This was an interesting experiment and a learning experience. 

[PRR]

> Added spec cathode bypass cap 25uf

? ? ? You do NOT bypass the cathode on a self-split stage. That kills the drive to the other side.

The AU7/BH7 *will* be a literal finger-burner when pushed to maximum rated dissipation. I had a coke-bottle shape burn from a 6550 run full-heat; the little twin-triode will run nearly that hot when at max rating.

[DummyLoad]

wouldn't a 6BX7 be a better choice? or4 6BL7?


--pete


EDITED. -pm

[frankenxtein]

To answer wouldn't a 6BX7 be a better choice?
Well this Pretty much balanced 12BH7A only cost me $10 



Thank you PRR

I just changed out this to the other chassis with the Voice of Music VM733 PT it had the bypass cap still on it cut that out thanks ..... More Clean 
Not too bad , pretty quiet

V2 (12BH7A RCA)
Plate 354.9v
Cathode 15.97v
675 ohm cathode resistor
(15.97 Divided by 675 = 0.023659 X 354.9 = 8.39)

V1 (12AT7 JJ) with 25uF bypass caps
P1 146.v
P3 2.18v
P6 142.8v
P8 2.22v

 Then I changed cathode resistor to a 817 ohm

V2 (12BH7A RCA)
Plate 361v
Cathode 16.85v
817 ohm cathode resistor
(16.85 Divided by 817 = 0.02062 X 361 = 7.44)

I wonder if that's close enough?

[frankenxtein]

Removed V2 470k grid stopper and 1M pot - Replaced with  500k pot
 
Recorded some disjointed noise. The Guitar is a $150 Charvel DS1 (2 knobs) MF special has  lots of sustain. Hot pick ups
Straight in to the head thru my H&K Attax 80's 12" 8ohm 
Just audio ..... iKey HDR7 (xy condenser no pad) 1 foot in front of the cabinet.
 Unprocessed except 24bit 48kHz down to mp3  high output pickups. 
        Sorry not much for playing these days  so it really sucks.
The beginning is everything set at 12 o'clock not sure if I kick the bright switch on and off. Then I just max everything it didn't catch fire.   

 It's very quiet , I'm half deaf so I couldn't say if it has "tone" but sounds good to me. I want to put some switches on the bypass caps. It's a nice tiny amp and would be useful to someone with a "Studio"  , versatile .   
https://www.dropbox.com/s/b9egjb3fp9oho4a/iKey_007.mp3?dl=0

[frankenxtein]

Made a cabinet for the Deluxe Micro .... painted the wrong side of the plexiglass face plate 

Too lazy to cut another so , it's gonna stay ugly