2x6V6 push-pull power amp - Fender Deluxe reverb copy

[dobrota]

I'm building guitar amp, power amp section is Fender Deluxe Reverb copy - I only change input capacitor from 1n to 22n to get more bass and mids. Power tubes are 6V6GT Electro Harmonix, output transformer is P-TF4131 from Antique Electronics Supply. Plate voltage is 378V and my bias setting is (using 1ohm resistors on each power tube cathode) about 24.5mA including screen grid current, so about 22.5mA plate current or say about 60% max tube dissipating - i guess not too cold and not too hot. All other voltages are OK and power amp is working without any problem, but I don't like sound - it's very thin, too trebly with strange almost acoustic sparkling when I play my electric guitar. With Celestion or Eminence Legend speaker, with various preamp design, all is almost the same, I think not good sound for electric guitar. I don't know if it's normal for this Fender model or 2x6V6 push-pull class AB power amp, because of I used more EL84 power tubes before in my gear. Please can me advice what's wrong ?

[wizard333]

Not normal. That is generally a very "midrangey" power section.

That plate voltage is very low for what a real Deluxe would run, most I've seen run in the 440v-480v range (yep, you read that right).

Also, unless you are severely limiting the low end in the pre-amp, that 22n is going to cause some farting out in the bass or blocking distortion with the volume and bass turned up very far. Thuddy pick attack is also likely at any volume unless the bass is turned way down... The silver face 10nf value is plenty big to feed that power section, some would say too big depending on the pre-amp. I find the blackface 1nf value to be too thin unless you're turning the amp up well into distortion range, though 10nf with the stock circuit can be almost overwhelming in the low end. Even with a Marshall style pre-amp feeding that power section, 22nf is going to lead to some mud in the bass unless you keep your bass knob pretty low.

http://ampbooks.com/home/amplifier-c...ing-capacitor/

Remember the lowest fundamental your guitar produces is 82hz. Still think you need 22nf?

If you're finding it thin and reedy, might be partially that plate voltage, also check your NFB, and check your pre-amp design. Have you used that same pre-amp at the same voltages with other power sections and got very different results?

[dobrota]

2x6V6 push-pull power amp - Fender Deluxe reverb copy

Thanks for your answer. I know my plate voltage isn't so high as in 65' Fender original, but I don't think it can change frequency response so much. Maybe OT which I've used (they say ..replacement for Deluxe and Deluxe Reverb, 8 ohms. Equivalent to #125A1A & #041318) wasn't right choice.
I used 22n cap because of it's usual value for full frequency response in various guitar power amps and preamps. I've checked NFB etc (power amp circuity is very simple) more times but I didn't find anything wrong. I also tried external guitar tube preamp in front of this power amp and sound is still similar - thin, acoustic, something like one plays electric guitar through hi-fi amp and hi-fi cabinet without guitar speaker.

[loudthud]

Set your DVM to AC Volts and connect it across the speaker terminals. What is the reading with no signal going through the amp?

[dobrota]

I measured it before, when I tried to find best OT position. There's little hum without singal on speaker output and about 18mV AC.

[uneumann]

Yes - the basic DR power amp does sound flat and uninteresting. It' designed to be essentially a flat-response power stage. Until you get some clipping at high levels, there's not much going on there that contributes to "tone". Below clipping, the tone-quality comes from the pre-amp... the DR tone comes mainly from the scooped response curve in the tone section. What preamps have you tried?

[loudthud]

I measured it before, when I tried to find best OT position. There's little hum without singal on speaker output and about 18mV AC.

In that case it's probably not oscillating although some amps won't break into oscillation until a signal is going through the amp. That can usually be cured with changes in lead dress. Have you tried the amp with the feedback disconnected?

[wizard333]

I used 22n cap because of it's usual value for full frequency response in various guitar power amps and preamps

Its normal for a Marshall running EL34s or 6L6s. It's also overkill even there and many of those amps benefit from cutting that back. .01 is normal for a Silver Face Fender running 6L6s. .001 is normal for a Blackface Fender running 6L6s, and no one accuses them of being thin or shy on low end. But you don't have those, you have a Deluxe power section running 6V6s.

Don't rely on what is in some dissimilar amp for a guide. There are a lot of useful calculators out there, quite a few even on that site I linked, use them as a guide. You don't need essentially flat response down to below 10hz, because 1) your guitar cant produce those frequencies anyway, 2) your speakers can't produce those frequencies, 3) a Deluxe Reverb power section can't deal with them.

Even a .0047uf PI input cap produces a response down to the bottom of the guitar spectrum that doesn't even drop 3db between 1khz and 82hz, which means for all intents, you can't hear it. All you're eliminating is low frequency noise and oscillation, like trucks going by outside etc, the thud portion of the pick attack, blocking distortion, etc.

[Bruce / Mission Amps]

...

Even a .0047uf PI input cap produces a response down to the bottom of the guitar spectrum that doesn't even drop 3db between 1khz and 82hz, which means for all intents, you can't hear it. All you're eliminating is low frequency noise and oscillation, like trucks going by outside etc, the thud portion of the pick attack, blocking distortion, etc.

Yeah, stock, I think the -3dB point is more like 15-20 Hz in the the long tail phase inverter... fairly flat from there on up.

In the classic BF or SF Fender modded circuits, I virtually never use anything more then a standard, plastic, 400v-630v .0022uF cap... well, maybe .0047uF cap if the guy only owns one single wound coil pickups guitar with unspectacular output.... ... and quite a few times I've also reduced the .1uF coupling caps of the driver to .033uF to .047uF.

[uneumann]

You don't need essentially flat response down to below 10hz, because 1) your guitar cant produce those frequencies anyway, 2) your speakers can't produce those frequencies, 3) a Deluxe Reverb power section can't deal with them.

Even a .0047uf PI input cap produces a response down to the bottom of the guitar spectrum that doesn't even drop 3db between 1khz and 82hz, which means for all intents, you can't hear it. All you're eliminating is low frequency noise and oscillation, like trucks going by outside etc, the thud portion of the pick attack, blocking distortion, etc.

OK - I'll get into trouble for this... but I'm feelin' feisty. This is the common wisdom about LF energy in guitar amps. Everyone says you don't need to pass anything below ~80hz. I buy that as long as you're strumming or picking notes cleanly. However, what about when you palm-mute the strings or want some pick thud for percussion, what are the frequencies actually produced by the guitar? I'm looking for some real data here... has anyone looked at the spectrum of some actual guitar played for LF percussive effects? (Think chunka, chunka... and yes, I know I'm encroaching on the domain of the bass player.)

There are two questions here...

1 - what is the spectrum of lower frequencies actually produced by a guitar? How low does it actually go?

2 - if there is any energy below 80Hz, is any of that spectrum musical or useful in some way?

One does not imply the other. The answer to #1 should be a fact, not a subjective response... either there is energy below 80Hz or not. The answer to #2 is more debatable, and I suspect there would be people on both sides of that. However, if everyone designs amps with 80Hz cut off, how will we ever know?

In thinking about subwoofers - those freq's contribute to a gut feel, rather than actual tone. I'm speculating that a guitar at high volume may produce a similar feel - due to very low freq's. I could be wrong, in which case I'll keep using the 80 Hz rule... or... there could be something we're commonly missing. Does anyone have real test data evidence?

[wizard333]

Yeah, stock, I think the -3dB point is more like 15-20 Hz in the the long tail phase inverter... fairly flat from there on up.

In the classic BF or SF Fender modded circuits, I virtually never use anything more then a standard, plastic, 400v-630v .0022uF cap... well, maybe .0047uF cap if the guy only owns one single wound coil pickups guitar with unspectacular output.... ... and quite a few times I've also reduced the .1uF coupling caps of the driver to .033uF to .047uF.

Yeah the only reason I'll go to .0033 or .0047 instead of .0022 or less is if I'm putting a gain channel where the non-verb channel was. Then I can just, just barely make out a fuller low end from the slightly larger cap, but its subtle at best, and probably only makes a difference in a 1x12 open back. Anything more than that and you wouldn't need it.

has anyone looked at the spectrum of some actual guitar played for LF percussive effects? (Think chunka, chunka... and yes, I know I'm encroaching on the domain of the bass player.)

Too much low end in either an early pre-amp stage or your power stage is the mortal enemy of effective "Chunka Chunka". For Chunka chunka to sound tight and percussive, that has to be limited. Too much lows in either spot makes Chunka chunka into "thunk thunk wet-fart wet-fart" instead. The low end just kind of goes BBBBBBR instead of that satisfying "SHHHUNG". I realize that may be a somewhat silly description, but I'm also pretty sure you know exactly what I mean if that is what you are into.

[uneumann]

Too much low end in either an early pre-amp stage or your power stage is the mortal enemy of effective "Chunka Chunka". For Chunka chunka to sound tight and percussive, that has to be limited. Too much lows in either spot makes Chunka chunka into "thunk thunk wet-fart wet-fart" instead. The low end just kind of goes BBBBBBR instead of that satisfying "SHHHUNG". I realize that may be a somewhat silly description, but I'm also pretty sure you know exactly what I mean if that is what you are into.

I guess you raise a 3rd point I didn't really think about - which is whether current amps can perform well enough at low freq's for an honest evaluation of low freq's from guitars and whether they have musical qualities. I'm pretty sure I know what you mean about those bad effects and I agree that most amps produce pretty horrible sounds down there.

However, you have to ask... are these bad effects due to the true signals from the guitar (?) or are they due to poor amp performance at those freq's because the amps weren't designed to perform well at low frequencies?

For example, most of us know farting when we hear it and yes it's bad, but it's often due to DC bias shifts or poor power supply performance, not the guitar signal itself. What if you built an amp that actually performed well at 10Hz? Wouldn't get get a heart throbbing pulse of LF energy hit you in the gut like a good bass amp produces with a low bass note or string slap? We know from bass guitars and amps, that percussive low sounds *can" sound good and there is musical quality in that range. Low freq's don't have to fart. Why do we put up with it and just say "roll off at 80 Hz"? Instead ... we could build better low-end amps.

[wizard333]

I hear what you're saying, but all you're reproducing below that fundamental is noise. Noise isn't music.

The Ampeg SVT with the cabinet designed for it produced very little in the range of the fundamental of the low note of the bass, but it sounded like God's gift to bass players. Not IN SPITE of the fact it didn't do that, but BECAUSE it didn't do that. The overtone sequence is what presents itself well in a mix and your ear fills in the gaps psycho-acoustically.

There are amps that reproduce very low frequencies well in a linear fashion, they are called hi-fi amps. Get a tube hi-fi amp and plug your guitar through it. Like what you hear? Like the feel? Try it and see, they do what you're advocating.

It takes a lot more power to amplify low frequencies than high frequencies due to the way our ears perceive sound. Amplifying a lot of low end noise sucks power out of your amp for all other frequencies. It's why low end oscillation is bad.

If you really want to amplify noise below the fundamental, get a powered subwoofer and run it from your pre-amp out along with the amp's normal output. It can amplify all the garbage your amp is rolling off, and it wont suck the power out of the power section of your guitar amp or create bad artifacts while doing it. IIRC Rivera makes one designed to attach to your guitar amp.

[loudthud]

In the last year I was messing with a JFET preamp that used an opamp servo to set the JFET bias. The problem was a peak in the very low bass, usually between 5Hz to 10Hz of 3 to 6dB. The source of the circuit I was trying to work with came from the Sunn Stinger 60. The schematic is posted on Ampix, but I can't access it just now. Anyway, the circuit just didn't sound right with that low frequency bump. The most practical fix is to roll off the bass at the input to the stage, and I didn't want to do that so I abandoned that circuit.

The key to good sounding bass seems to be a slow dropoff in response staggered between stages, not all the poles right on top of eachother causing too fast a rolloff. A shelf type response works in many cases.

[uneumann]

I hear what you're saying, but all you're reproducing below that fundamental is noise. Noise isn't music.

IMO - that seems more opinion that reasoned. A string slap or pick impact has a whole range of freq's in it. One man's "noise" is another man's percussion. I don't think we should call drums "noise" just because they are non-harmonic. I'm OK to say it's a matter of opinion and taste so let's not dwell on the word here.

The Ampeg SVT with the cabinet designed for it produced very little in the range of the fundamental of the low note of the bass, but it sounded like God's gift to bass players. Not IN SPITE of the fact it didn't do that, but BECAUSE it didn't do that. The overtone sequence is what presents itself well in a mix and your ear fills in the gaps psycho-acoustically.

OK - interesting point. But we're not talking about fundamentals under 80Hz anyway - as pointed out above... The low stuff is more percussive in nature.

There are amps that reproduce very low frequencies well in a linear fashion, they are called hi-fi amps. Get a tube hi-fi amp and plug your guitar through it. Like what you hear? Like the feel? Try it and see, they do what you're advocating.

Definitely a good point. I may play with this some using SS power amps. I wish I had tube hifi power amps lying around but not the case. I'd need to A/B it through the same preamp, and cab - although that's a potential problem... most guitar pre-amps and speakers don't handle lows by design. I'll have to think about it some... maybe a bassman amp can work for this. I recall the preamps on those are pretty basic and they don't have small coupling caps to limit low response. The 4x10 speakers may also work pretty well. I can just switchout the power stage... (just thinking out loud)

It takes a lot more power to amplify low frequencies than high frequencies due to the way our ears perceive sound. Amplifying a lot of low end noise sucks power out of your amp for all other frequencies. It's why low end oscillation is bad.

The power issue is definitely something to consider. I wonder what the energy is below 80Hz and just how much it will soak from an amp. Again, I wish we had some real spectrum data. It would help to know what's really going on instead of just guessing or taking someone's word for it. Again, real data anyone?

If you really want to amplify noise below the fundamental, get a powered subwoofer and run it from your pre-amp out along with the amp's normal output. It can amplify all the garbage your amp is rolling off, and it wont suck the power out of the power section of your guitar amp or create bad artifacts while doing it. IIRC Rivera makes one designed to attach to your guitar amp.

This seems like a great option - assuming there is useful stuff below 80Hz. You could turn the LF amp on/off and leave current amps alone.
Great point - I'll look into this... it does intrigue me that someone else (Rivera) thought the LF signals are worth building a whole product around.

Thanks for your thoughts...

*** quick edit - I've looked at the Rivera site and I can't find anything like you've described... can you point me to it? Thanks.