How to make my 5F2H a little Browner

[slohand42]

Hello,
I've lurked in and out of these forums for quite some time now. I have completed a build on a 5F2H, completely to the published schematic and parts. I really like the little amp. I've played around with speaker choice (10", 2 x 8", different 12"s) and pre-amp tube swaps to get the tone I want the best. I'm very close, but I would like it a little "browner". To be honest, I first hooked up the 8 ohm speakers from the 4 ohm lead. Definitely less power, but a much browner tone (browner and mushier than I liked). When I found my mistake and corrected it, the amp came much more alive. It really barks, but it is now a bit too crisp for my taste.

I would like it to be just a touch browner, or greasier if anyone really knows what that means. Obviously I will revisit my collection of pre-amp tubes, but are there any suggestions that may give me an easy tweak to get me in that direction.

This is my second actual build. The first, of course, was a nice 5E3.

Thanks,

Ted

BTW--I play through a stock JT-30 with an early, but hot crystal element, a JT-30 with a Shure Black Label, a Shure Bullet with a hot White Label, and various other mics I have laying around.

[MWJB]

What's the voltage on pin 1 & 6 of the 12A#& tube? This can be reduced by increasing the value of the 2.7K resistor between the screen supply & the preamp filter cap...doubt I'd go much lower than 130-140vdc on a 12AX7 (lower mu tubes will return less voltage)?

If you're really "nearly there" try removing the 25/25 bypass cap from pin 8 of the 12A#7 tube (leave the resistor in place) and/or try removing the bypass cap from the KT66 cathode resistor...if you do both you may end up with too little gain, so try one at a time. With the cap removed from pin 8 12A#7 this would allow you to try a NFB loop to take the edge off respose a little, maybe a 33K from the ungrounded end of the cathode resistor to the hot speaker terminal (if the amp squeals reverse OT primaries or secondaries).

...or you could try replacing the 2.2K cathode resistor at 12A#7 pin 8 with 1.5K or 1.8K?

All suggestions are just that, hopefully Bruce will chime in.

[Bruce / Mission Amps]

Is it one of the Weber kits or did you source everything yourself?
Try a 39K resistor instead of the 27K (R14) resistor in the B+ rail... or solder a 1000pF to 1500pF cap across one or both of the R17 and R15 150K plate load resistors (one at a time) of the preamp tube's triodes, and use a .01uF cap for C3.

[MWJB]

As Bruce points out the preamp dropper is 27K (R14) NOT 2.7K as I had incorrectly advised.

[slohand42]

Thanks, guys. I know you two are pretty knowledgeable about all this.

To answer some questions, I did source the parts myself, though I built it to the schematic posted on Weber's site. I even made the board (eyelet style), the chassis, and the cab. I used a Hammond PT and I believe the choke was a Hammond as well. I used Mallory and sprague caps. A VA KT66 and a GE 12AX7 from the 60's. Bruce may be dissapointed in one choice. I did get the OT from Ted Weber.

The speakers I have been working with are mostly alnicos. Magnavox, CTS, Jenson, etc., with smooth and ribbed cones. The one that sounds the best so far I believe is a Magnavox gold something or other 12".

The plate voltage on the preamp is 145, which can obviously come down. I'll probably try the 33K resistor first. I may experiment with the "snubber" caps across the plate load resistors etc.

I don't have all of that stuff in stock, so will have to wait until tomorrow at the least.

Thanks again for your advice and input. I will report on how it turns out.

Ted

[slohand42]

For a follow up, I did scrounge up some resistors to drop the B+ voltage (I've got to clean off my bench). The 39K resistance brought the B+ down to 135.
Testing this setup showed good promise, but I wanted just a hit more brown, so I upped the resistance to 54K. This brought the voltage down to 129. All I can say, is WOW! With this, the amp really came alive to my taste, and the tone circuit seemed to be much more effective. Feedback was not a real issue, and the variability that was available with different cupping (from airtight cup to wide open) and "attack" techniques was great.

To be completely honest, the 129 volts just barely tipped over the edge of where I want to be, so I will raise the voltage somewhere between 129 and 135. That should really dial it in for me.

I know Gerald Weber said he went with 80-90 B+ volts on his harp amp. I tried that on another amp and all I got was mud. To my ears there's a big difference between greasy brown and mud.

Once again, Thanks for the information and help.

If you have more input, I am very open to hearing it.

Another Way Cool harp amp has been born.

Ted

[MWJB]

"I know Gerald Weber said he went with 80-90 B+ volts on his harp amp. I tried that on another amp and all I got was mud. To my ears there's a big difference between greasy brown and mud." I would usually find 80-90v too low on a 12AX7 too (I have tried the Kendrick amp, the more grind channel runs even less voltage)...though it does vary with speaker & output tube configuration (I've gone lower on some amps with acceptable results) but for an amp like this I'd be typically expecting 130-170vdc.

[Bruce / Mission Amps]

I'm using the 39K on mine with a 5V4GA, an old, well used, black plate 12AX7 and a KT66.

The Weber SE 15 OT should be fine... unless something has changed over the last few years, ... it was a Chinese version/copy of a Hammond 125ESE OT, which I personally sent Weber in order to see if his Chinese winder could make a clone of it.
They seemed to have done a decent job on that one.
However, I would not run anything over a single KT66, 6L6GC for best results... it only rated at 80ma DC current so keep that in mind.

http://www.hammondmfg.com/125SE.htm

I'm considering making one with two parallel KT66s and a Hammond 125GSE but shoving it into a tweed Vibrolux chassis/cabinet with a single 12" Alnico.

[slohand42]

Thanks. When I get this one dialed in, I won't be messing with it any more, so the OT should be fine. If I want to play around some more, I'll just start another one, or work on the Bogen K10 I have. It has a pretty sweet tone to start with.

[arnenym]

I'm considering making one with two parallel KT66s and a Hammond 125GSE but shoving it into a tweed Vibrolux chassis/cabinet with a single 12" Alnico.

I believe i'm disqualified to call my amp a 5FH2, But its still 90% that circuit
I done my variant of this amp with two sovtek 5881 wxt a hammond 125 GSE 25W OT and a Hammond 370?X PT, no choke.
I found out a Eminence Alpha 10" is a very good speaker to harpamps, but maybe a little dark to this circuit.

[Bruce / Mission Amps]

I believe i'm disqualified to call my amp a 5FH2, But its still 90% that circuit
I done my variant of this amp with two sovtek 5881 wxt a hammond 125 GSE 25W OT and a Hammond 370?X PT, no choke.
I found out a Eminence Alpha 10" is a very good speaker to harpamps, but maybe a little dark to this circuit.

Go ahead and call it a 5FH2... mine is called a 5F2H

[6267]

I had an interesting experience with putting resistors in the center tap of the OUTPUT transformer, someone mentioned that in the theory and design section a while back. As long as you don't drop the B+ so much that the anodes are lower than the screens, it has a pretty neat tonal quality. Use a big resistor. This resistor sees a lot of current, so I'd guess high and double it for reliability. It could probably be made switchable somehow, but definitely gave a "browner", swirlier sound.

[slohand42]

I've put zener diodes on center taps to drop the total output voltage of the PT, in fact I did that on my 5E3. It worked great. I've never tried just resistors. I actually tried the zener diodes on this 5F2H. It dropped the B+ voltage, but I was happier with the tone and color by just raising the B+ resistor value.

Has anyone else tried just a resistor on the CT?

Ted

[slohand42]

My B+ "Browner" conclusion--so far.

I have come to a conclusion concerning the B+ value. I honestly love the tone that I get with a 54K resistor on the B+ rail (120V B+). It is brown, honky and all sorts of good things. I know a lot of people talk about Little Walter's tone, but to my old ears, this honestly approaches it. Since, however, I don't always want that tone, and with that B+ voltage, the amp is limited, I've decided to make the B+ voltage switchable. Probably between this 120V and approximately 130V. Both are awesome sounds and the switchability will make the amp that much more versatile.

I'm likin' it.

Ted

[slohand42]

I'm using the 39K on mine with a 5V4GA, an old, well used, black plate 12AX7 and a KT66.

Bruce,

I'm pretty uneducated in the differences in rectifier tubes. Is there much performance difference between the 5V4GA and the 5Y3?

BTW, the best tone I could get in my amp came from some old Silvertone 12AX7's from the 60's. Don't know who made them, but they were made in Holland.

Ted