Are these voltages too low?

[markmalin]

I'll attach a schematic below.  The previous version of this amp used a Hammond 290DX but this time I went with Mercury Mag trannies, so I used the FBFS P40X.  Both spec out at 325-0-325.  So here's how the voltages compare between the two amps:

MM trans version

V1a plate:   189
V1b plate:   187

V2a (O.D. section) plate: 170
V2b (O.D. section) plate: 178

PIa plate: 185
PIb plate: 185

KT66 plates:  412

GZ34S rectifier pin 2: 412V
GZ34S rectifier pin 4 (input): 320 VAC

Hammond trans version

V1a plate:   236
V1b plate:   230

V2a (O.D. section) plate: 200
V2b (O.D. section) plate: 200

PIa plate: 200
PIb plate: 200

KT66 plates:  442

GZ34S rectifier pin 2: 485V
GZ34S rectifier pin 4 (input): 352 VAC

I guess what I'm wondering is, are these voltages for the MM trans version (top one) too low?  And if so, what options do I have, other than using a different power transformer?  The amp sounds OK, but I need to mod the bias pot resistor to get enough current...right now I can only bias the KT's at 30ma so it's sounding a tad anemic.  I've got a guy coming to pick the amp up Sunday so I've got to figure this out...or determine whether it's not really an issue :)


Thanks guys!  As always, I humbly appreciate your input.


Mark

[Willabe]

You have to consider current rating also.

A PT with higher current rating won't drop as much (sag) B+dcv when it's loaded with all the tubes in and are drawing current.



                  Brad     

[jjasilli]

I think you're good to go, after re-biasing.  Per Willabe's post, though the PT's both spec out the same as to voltage, the MM probably has less current handling than the Hammond, so the same load produces a larger voltage drop.

I don't see anything wrong with running KT-66's @ 412V, which is within 10% of the prior 442V. 


The lower small bottle voltages downstream may be fine; or can be restored to their higher values with smaller dropping resistors in the B+ line.  But maybe the PI voltages should be restored, which will boost all downstream voltages anyway.

[tubenit]

I thought this was interesting enough of an idea that I wanted to draw a schematic for it.

I had difficulty reading some of the component values on the schematic that Mark posted, so this is my best guess.

There is an editable SCH version that can be modified for corrections and mods.

With respect, Tubenit

[markmalin]

Wow!  Tubenit, thanks!!  I have to look the schem. over to see if the values are correct.  Sorry the original is so bad, it's literally cut/paste/whiteout.  I'll take a peak at it tonight if I can find the time and make any corrections.

The actual amp had 2 10K Ohm resistors in the power supply filter dropping string, so I changed one to a 4.7k per the schematic and the plate voltages are much more in line.  I don't have them handy but can post later this evening.  The KT66 plates are still sitting at 410V, but the pre-amp plates are more in the 200 range

I made a couple changes in the circuit to tame the reverb down.  It was insanely lively and only usable from about 1 to 3 at the max on the Reverb control pot. I changed the values of the 3.3M mixer resistor, as well as the 470K in the Reverb circuit.  I will post those changes, but essentially I lowered the value of the 3.3 M to around 1.5M and upped the 470K to about 1M.  It's WAY more reasonable now - very nice, in fact.  You can dial it up to about 5 and it sounds nice and lush.  Over toward 10 and you've got Surf music, but 5 and below is very very nice.

The other unique thing about this circuit is, the 3 tone controls seem pretty interactive.  I've been trying to tame the base, but I find that if I set the Mid pot to ~6.8k, like on a standard DR or other non Mid pot type Fender tone stack, the treble and bass respond nicely.  Turn the mid down and you can get very trebley...or very bassy, which makes for a nice jazz sound.


The one thing I'd like to change is, the OD is rather muddy unless you turn the bass down quite a ways.  Granted, it's kind of an old school Clapton sound (overdriven Fender...not very nice IMHO) but with the bass down and the treb and mid up and the O/D engaged it's nice and "dumbly".  In any case, I may try to brighten up the OD a bit - just my taste, I guess.

Another unique thing is having the pre PI master.  It works nicely in that it quiets down any of the faint "hiss" you get from some AB763's.  But I also find if you play with the balance between the Master and Volume controls you can get nice punch at low volumes, or nice clean non-punchy jazzy tones.

Sorry to ramble -- thanks for the help everyone.  I still have to tweak the bias pot resistor to get the KT's idling above 30 ma...then I should be done.


(Incidentally, if anyone's interested if you search instagram for #BadDog or #MalinAmplificaiton I'm always posting pictures of my builds  )

[tubenit]

Mark,  don't know if you've ever tried it but I use a "sort of" 5E3 tone stack on my OD channels of the D'Mars and the Tweed BluezMeister.  It helps me dial in a wider range of tone shaping for my OD channel.  I find it useful.

With respect, Tubenit

[markmalin]

Mark,  don't know if you've ever tried it but I use a "sort of" 5E3 tone stack on my OD channels of the D'Mars and the Tweed BluezMeister.  It helps me dial in a wider range of tone shaping for my OD channel.  I find it useful.

With respect, Tubenit

So you add another tone stack, just for the OD?  Or do your amps have an entirely separate OD channel vs just switching in the D-style gain stages like my example?   Now you've got me curious ;)

[tubenit]

You can use either 390p, 500p or 680p per your taste in tone.  I've done all 3 in different amps.

And on the "bottom end",  I've used either .0047 or .01

As an FYI,  I enjoy and appreciate your posts on the Hoffman forum.  Thanks for sharing them.

With respect, Tubenit

[markmalin]

Wow, that's brilliant.  I may try that out - I'm thinking just keep the pot internal and attach the few components to the open area on the board.  Then I can set it and forget it.


Thanks, Tubenit!


Mark.

[tubenit]

When DaGeezer does the tone control on his OD,  I think he uses an actual vs. pseudo 5E3 tone stack?

With respect, Tubenit

[markmalin]

I've got about a day and a half before the customer comes to pick up the amp, so this AM I rigged up a 1M pot with the 2 caps soldered to it and some long leads.  If I have time tomorrow I"m going to temporarily solder this in and see if I can dial in a less muddy OD tone.  If so, I'll just measure the pot value and hard-wire a resistor and the 2 caps in there.


Thanks for the schematic, by the way.  It was really interesting looking at how you did that circuit.  Oh, and I still need to check my drawing.  If I can figure out where to download the schematic program I'll make the changes.  Just a few values are different (thanks to my multiple re-copying old schematics and using whiteout and Paint.NET ;)


Mark.

[Willabe]

If I can figure out where to download the schematic program I'll make the changes.

Here's the link;

http://el34world.com/Forum/index.php?topic=590.0


                  Brad   

[tubenit]

Mark,

I redid your SCH schematic that I previously posted and added the PT,  rectifier and B+ rail.  See attached

Should be ready for your editing which is very intuitive and just clicking on a lot of stuff.

Tubenit

[markmalin]

Mark,

I redid your SCH schematic that I previously posted and added the PT,  rectifier and B+ rail.  See attached

Should be ready for your editing which is very intuitive and just clicking on a lot of stuff.

Tubenit

Tubenit...you're just too nice!   Thanks, this is great!!


Mark