first build...It works (!!)...but....

[pula58]

I designed and built an amp that is kinda like a super reverb, but with no tremelo, just one channel, and a tube recitfier.


2 6L6 output tubes, 2 12AX7's and two 12AT7 (PI and reverb driver).

Anyhow, it basically works fine after I clear-up a few unconnected wires!

But..the tone is somewhat lacking...so..I have a few questions

1) what would lowering the plate voltages on the first (v1) tube do in terms of tone? I have seen people put a 150k load on the "D" (lowest) preamp supply voltage to lower this voltage a bit more. What is the toanl effect of this?

The basic thing I'd like to do is make the amplifier less "ice-picky" and more full sounding.

I would also like to make it so that the reverb doesn't splash when I take the amplifier off stand-by.


I used Xicon polypro caps almost everywhere..except some orange drop "PS" series caps. at output of second triode of V1, and a few other places.

What makes the highs kinda ice-picky and NOT sweet?

I used a Hammond 291EX (blackface bassman) power tranny, a hammond choke (twin reverb/super reverb size) and a hammond multi tap OT (abassman size, 4200 ohm primary, 2,4 and 8 ohm output Z).

Any good ideas to sweeten up the Fender sound?

[selloutrr]

what brand of tubes are you using.
paper oil tone caps would help.
what other adjustments have you made to the circuit? i.e. parts values.

[pula58]

here is a schem (caps marked with orange box are orange drop "PS" series caps.

The 0.1uF cap in the tone stack is a Mallory 150.

the 250pF cap i nthe tone stack is a silver-mica

The rest of the coupling caps are Xicon MDD (I think they are polypro) caps.

All R's are metal film 1/2W except for as noted in the power supply and 470 ohm and 1.5K ohm R's at output tubes. oops, I lied, there is a 100K carbon comp cap at the last plate (v3) before the PI.

Winged-C output tubes biased at 21W each
V1: GE NOS 12AX7
V2 (reverb driver) is an old NOS tung sol 12AT7
V3 is tung-sol 12ax7 (new)
v4 is NOS 12AT7 (Seimens?)

Reverb tank is a 4AB3C1B

[selloutrr]

ok did you meter the resistors before you installed them or just put them in? the reason i ask is it's not common but still possible a resistor is out of spec.
I can't view the schematic on my phone i'll look when i get home

[pula58]

I measured all the resistors before I put them in. Plus, they are 1% metal film R's (mostly). The 100K carbon comp is real close to 100k (like maybe 99K or something like that).

Thanks!

[d95err]

You haven't mentioned the single most important part for tone - the speaker. So, what speaker are you using? Have you tried different speakers?

[Firestorm]

1) what would lowering the plate voltages on the first (v1) tube do in terms of tone? I have seen people put a 150k load on the "D" (lowest) preamp supply voltage to lower this voltage a bit more. What is the toanl effect of this?

Increasing the plate load won't drop the voltage. You'll just increase the gain and the range of the max output signal swing. The preamp node resistor in the dropping string is what you manipulate and I see you've already bumped it to 7K5.

As d95err says, what speaker(s)? Where have you got the feedback loop connected (I see you reduced the shunt resistor)? Is it too bright with the reverb on or all the time? (I see you've changed the reverb circuit -- very sensitive part of the amp).

The brightness in this circuit is controlled by the 3M3/10pF coupling the reverb recovery stage. If the 10pF is over spec, the amp gets brighter. Some silver micas have pretty sloppy tolerances. You won't be able to measure a cap that small unless you have a dedicated capacitance meter, so maybe try some swaps.

You can also change the .001 cap into the PI to change the balance of highs and lows. Make small changes. Try .002, .003, .005. Gerald Weber used to recommend a .02 here. Do NOT go that high if you have 10" speakers.

[pula58]

When I mentioned lowering the plate voltages in the preamp I meant to either A) increase the value of the 7.5K resistor in the power supply string

-or-

b) put a 2W 150K resistor from power supply point "D" to ground.

I am using a 250K reverb "mix" pot (audio taper) to make the reverb easier to adjust (the fender linear pot is kind-of abupt)

since I am using the 250K reverb mix pot, I decreased the coupling cap to the top of the reverb mix pot to 0.0012uF (1.2nF). I have a Fargen amp that uses the 100K reverb mix pot and a 0.002 cap there, and it sounds great. So, since I doubled the pot value (approximately) I thought I'd back-off on that cap value a little.

I am running the amp into a bunch of different JBL's. But before you guys get-started talking about icepick and JBL's let me say that when I run my Princeton reverb into the identical speaker(s) it sounds much sweeter.


The thing I am trying to tweak is subtle, yet important. It has to do with the high-end mostly..I want it to sound a bit more sweet, and less sterile.

So, I am wondering about subtle things like capacitor type: Are the coupling caps I am using harsh sounding? Should I switch to a "sweeter" type of coupling cap? Increase the 7.5K power supply string resistor?

True..I have no way of measuring the 10pF silver mica cap....but the tolerance on the one I am using is +/- 5%. Same goes for the 250pF silver mica in the tone stack.

I have the feedback 820 ohm resistor connected to the 8 ohm speaker tap of the output tranny (it is a multi tap, 2,4 and 8 ohms, I am only using 4 and 8 ohm taps). The other feedback resistor (the on tha tgoes to ground) is 50 ohms (I changed it down from 100 after looking through some Fender schematics)

[M Fowler]

Why the 1m resistor into the bright cap? To reduce popping on the switch it should be wired different.

[telentubes]

delete

[pula58]

Why the 1m resistor into the bright cap? To reduce popping on the switch it should be wired different.

The 1Meg in series with the brite cap tames (to my ear) the tonal ffect of the brite switch. Makes it more useable for me.

[Firestorm]

Sorry, I misread your voltage dropping question. If the issue is frequency related, I don't think I'd start off playing with the supply voltages. A few things to try: carbon comps for the PI plate loads -- both the AC and DC voltages at this point are big enough to make the CCs "do their thing"; for the coupling cap to the PI, try the dreaded ceramic Fender always used, or a silver mica -- I've sometimes used metalized polys like the M150, but never film and foil like the 6PS so I don't know what effect that might have; some SRs use a .02 cap in the tone stack instead of a .047, but this was likely a 10-inch speaker tweak; you've pretty much replicated the signal loading effect of the tremolo pot, but the AB763 circuit also has a 220K series resistor into the PI which will affect higher frequencies.

[Structo]

The 100K plate load resistors with the 1.5K cathode resistors is the gold standard for tube amps.
It keeps the tube centered in a good operating window and should sound good.

You could try a larger PI input cap like a .01, a lot of amps use a .1uF there.

Try larger couplers on the power tubes like .01uF.

In fact, the .1uF on the second PI input is pretty high.
I would use a .02 there.

[Normster]

Ceramic for the treble cap for sure. I'd also give some thought to replace the 250KA master with a 100KA. Same with the reverb.

Since the NFB circuit is using fixed values, you might want to try a 1K trimmer in place of the 820R to see if you can dial in the amount of top end you're looking for. Another option is to add a hi-cut control either on the front panel or as a trimmer. It's just a 250k trimmer in series with a .0047uF cap between the your output caps (bias side).

You didn't say which tank you're using, but I find the new long delay tanks are pretty much worthless. I'm sold on medium delay. Much less splashy. You can also dial out some of the metallic sound by adding a tone control circuit. (Just a cap and trimmer off the reverb pot.) I think Allen amps uses this. I don't remember the values.

Lastly, and completely unrelated to your question, try grounding your bass pot through a 4k8 resistor so you can keep the mid pot independent of the bass ground. This allows you to use a much larger value mid pot without effecting the bass control. I like a 100kL mid pot. You'll still have the ability to scoop the mids like Fender, but crank it up for some Vox/Marshall mids.

[pula58]

The 100K plate load resistors with the 1.5K cathode resistors is the gold standard for tube amps.
It keeps the tube centered in a good operating window and should sound good.

You could try a larger PI input cap like a .01, a lot of amps use a .1uF there.

Try larger couplers on the power tubes like .01uF.

In fact, the .1uF on the second PI input is pretty high.
I would use a .02 there.

Oops, I forgot to show that, in the actual circuit, I am using a 0.047uF Xicon (polypro) Capacitor at the "2nd" input of the PI (the place where the 820 ohm/ 50ohm feedback resistors meet-up.