Pushing V2 harder within older fenders. Pre-amp clipping mod?

[Lydian]

Hello again mates,

So I'm expecting a few to tell me to leave the amp alone, since 'they' got it right the first time. This very well may be the case. Aside from that, I have an AA165 scheme 68' proreverb that I'm trying to push into clipping at lower volumes within the preamp stage. Most the gigs I play at won't allow me to play my amp in its sweetest spot, just too loud, so I'm trying to make due. Yes, I know I can throw and OD pedal in front but even that hasn't done the trick for me thus far.

I have been checking out vintage preamp tubes lately and have loved how some of them distort, soo much better than reissues and new stock. Has anyone ever experimented with adjusting the
bias of V2 in this situation? Ive read a small article that changing the 1500 ohm resistor and 25uf cap from V2 cathode to ground can bias the tube differently. Can you this be done in a suitable way in order to really get this tube to sweat and show its beautiful clipping capabilities at lower volumes? Such as volume 4 instead of 6-7? I'll be getting my hands on some more RCA's Raytheons Mullards hopefully soon. It would be nice to represent the beauty of these tubes live with my current setup.

In the long run I'd like an 8-15 watt tube amp that I can push hard for my small gigs but, 400 hundred bucks just isn't laying around right now. So it seemed to me a cap and resistor or 2 was a poor mans hopeful alternative. I look forward to any suggestions and opinions out there.

Btw I already have a NFB mod on the ground switch so that's out too.

Cheers,
Dalton

[Enzo]

The problem is your goal of more distortion by increasing the gain of the tube. That has the net effect of increasing the signal level, ie it will be louder.

If you want to crank the preamp, add a master volume before the power tubes. or similar project.

For a signal stage to clip, the signal level has to exceed the linear range of the stage. We have to overdrive the stage. That doesn't happen by turning it down.

I assume you have discovered that disconnecting the NFB makes the amp louder as well.

[pdf64]

Most any pre-amp will likely have way more clean output available than is required for the power amp to clip.
So tweaking pre-amp bias etc is unlikely to achieve pre-amp clipping; unless it's taken to unpleasant sounding extremes, which would also prevent the power amp reaching full power output (with the affected channel).
So as Enzo suggests, a master volume may be a better way forward, see p32 of https://robrobinette.com/The_Trainwreck_Pages.htm with the type 3 aka crossline being perhaps the most simple to implement, though a 250k audio pot may work better with a 12AT7 LTP.
Another more complex option is power scaling / variable voltage reduction (VVR) which allows controllable reduction of the the power supply voltages, thereby reducing the amp's power output eg Selecting a Power Scaling Kit

[Richard]

There was another thread about a Super reverb that had 2 speakers disconnected to lower the volume.

http://music-electronics-forum.com/t41416/

If you do this you will be running an impedance mismatch, I don't know if you're comfortable with that or how high you will be turning it up. Less efficient speakers may also be an option. Swapping in JJ 6V6's could help, I believe they like a higher output impedance so you could also disconnect a speaker.

I had my Princeton modded at one point and it seemed to get more breakup. I believe the guy changed the cathode resistor and bypass cap on the second gain stage. I believe both channels share the cathode resistor and bypass cap so this will affect both your channels and it probably broke up more but not at a lower volume. This is similar to your question about the bias of V2. However the 1500 resistor and cap is on the first stage which is driven only by the guitar. I doubt there is enough signal to take advantage of any changes. The second gain stages of both channels are tied together, I believe that this is the best area for this but any change will affect both channels. Pulling V1 will effectively change the bias of V2B. I tried this on my Super Reverb but it didn't seem to do much.

From what I've read on this site I would try disconnecting a speaker, and then the 6V6's. Perhaps a 5U4 rectifier may be an option?

[olddawg]

I play clubs a lot with 15 - 30 watt amps. The best thing I have found to hit that sweet spot without blowing the place up is using a 50 watt Weber Mini Mass attenuator. I even have mine set up with a footswitch to turn it on and off. It doesn't work well if you are totally squashing the output. But for taking the edge off a blaring amp in a smallish room and still cranking it, it works great. Last time I looked they were about $100 US and another $20 for the factory footswitch modification. I also modified the footswitch with an LED so I would know if the attenuator was engaged.

[J M Fahey]

Easiest/best is the cross plate MV suggested by PDF:

It allows you both to set all knobs to 10 to *make* the amp clip and then some, and dial output power down as much as you like,even bedroom levels.

To avoid drilling the front panel you can put that pot in the back or under the chassis, where you can still tweak that knob with a fingertip; signal there is quite high (tens of Volts) so not exactly sensitive to interference, a pair of regular wires, twisted as much as, say, filament wiring, is good enough.

Remember those wires (and pot legs) carry power tube bias voltage attached, so they may be at, say, -50V or so (DC)

[Lydian]

Doesn't seem too intrusive. I've been scratching my head on all the options to further hunt down the elusive tone for me. It's between this master volume mod or some new speakers. I've got a pair of warehouse speakers around 99 SPL, I'm thinking of switching to some Jensens P12Q around 95 SPL. Of coarse this will change the tone quite a bit too, it may give me more vintage rock sound.

[Chuck H]

What they said^^^^^^^^^^^^^^^^

But!.. Preamp clipping won't sound like power amp clipping. And, because of the topography of your amp, any clipping you get now is power amp clipping. The topography of that amp makes it impossible to keep it's signature sound and still clip the preamp appreciably. So, any mods you make to increase preamp gain are most likely to increase clean signal to the power tubes. That doesn't mean you wouldn't like the mods. It means that even though the tone may be altered, it won't be clipping at a lower volume.

Your best course of action, IMHO, is to decrease output efficiency by reducing speaker efficiency, reducing voltage, or employing an arttenuator.

Reduce speaker efficiency: The AA165 is shown as a two speaker amp in all the schems. Some Pro's were made with a single speaker. I don't know if there was any crossover, some of that stuff happened with Fender. If it's a two speaker amp I might suggest using one speaker. It would need to be capable of handling the full output of the amp and it shouldn't be an efficient speaker (below 97dB spl). As noted in the thread that Richard linked, it will also reduce efficiency to mismatch the load by 100% in either direction. The combination of reducing the driven cone area and the mismatching the impedance should reduce volume considerably. NOTE: DO NOT attempt to reduce volume further by mismatching greater than 100%. ie: a 4 ohm requirement into an 8 ohm or 2 ohm load. Altering the required load impedance by more than 100% in either direction can damage tube amps.

Reduce voltage: There are kits available that reduce operating voltages in the amp. And therefor, efficiency. They can be wired to affect only the power amp. This leaves the preamp voltages alone and better maintains the amps character. The effect is called "power scaling" and you can find numerous threads here on the subject.
You might also be able to add a "sag" resistor to the existing circuit that would reduce voltage enough to allow for the use of 6V6 tubes and more sag at lower current levels to simulate the sound you have now, but at a lower volume.

Attenuator: What can I say... There are a lot of good ones out there. olddawg mentioned the Minimass. That's a good one. They ALL sound pretty good if you're only taking a little off the volume. Most sound pretty bad turned down to bedroom levels. But just a little volume trimming for gigs isn't offensive with most of the offerings.

HTH

P.S. My vote is for the power scaling or an attenuator. Both of these allow the amp to be used as stock more readily and also allow the use the speakers you have now, which should better simulate the tone you know and love.

[The Dude]

Another thought (thinking out loud): You could replace one speaker with a totally resistive load. It would have to be able to dissipate the heat, so would be large and heat sinked. You could even make it switchable so you could use both speakers if you needed more volume.

[J M Fahey]

Agree that (fully) Preamp clipping is not the same as Power amp clipping, specially for Fendery type crunch.

But PPIMV is not that either, at worst I'd call it "half way there" ... you are clipping the PI , which IS part of the power amp, destroys the power amp NFB (if existing), what clipping the power amp, etc.

Not the same but quite close, much closer than any standsrd MV or pedal.

Try it, it's *easy* to test , you just need a pot (agree with the 250k Audio/Log suggestion ) and 2 pieces of wire, you may even leave the unmounted pot hanging from them ... for testing, of course.

If you don't like it, you pull it, the whole test takes all of 10 minutes and is as un-invasive as anybody might imagine.

Post results, one way or the other.

PD: switching to less efficient speakers or pulling one amounts to a still way too loud amplifier, a 3 dB change is very small.

For a *useful* yelling reduction mod I'd start with at least 6dB

The other option is to add a fixed (but switchable) resistive attenuator , just 2 power resistors (suitably mounted on a piece of metal or something, bolted to the combo wall or floor) and a switch.

Personally think the variable voltage Mod is the best, but I have seen so many *horrible* installations that I simply don't trust a Musician to do it properly, sorry.

Unless it's a very well made kit or something, not usually the case.

MosFets need to dissipate *way* more than what is typically used (I use robust IRFP450, which are widely available here) and a *large* and *well mounted* heat sink, sometimes add a fan, can't believe why many suggest a puny TO220 MosFet (---820) , plain bolted to an iron chassis, near very hot tubes, etc.

Oh well.

[Amp Kat]

Just my opinion but Imo the fender stages really don't create a good distortion tone because they were never intended to be a distortion amp. Leo built it basically off of Hi-Fi design into a guitar amp and the first rolloff is just to low to create the brightness to be a good sounding distortion amp. Now guys like Zinky did some serious work to get those amps to have a sweeter distortion but a lot of work and switching networks to bypass the original Fender tone shaping networks were used. On the flip side if you use a Marshall preamp out into a Fender it sound exceptional as does the Radial British pedal in front of the amp. The OP is looking for a sweeter tone in his Fender at a lower stage volume which Imo isn't going to be gotten with just an attenuator because there simply isn't the right type of clipping from the Fender circuit to make it sound like he wants.

Pedals in front are another option and if you get the right one can sound really nice with exceptional preamp tubes and Imo the Telefunken smooth plate is the best sounding tubes in those amps however I'm using some Baldwin 12AX7 blackplates that are really nice and they do help. Yet another option is to wire a tube up as a diode and use it just like the poor mans distortion technique that Peavey,Marshall and so many others use however reference voltages are important because of modulation swings in amplitude which is a natural phenomenon in preamp tubes however, what you get is a more rounder clipping wave than the sharp square that diodes and even the infamous JRC 4558D will give you. They are not easy to incorporate into your design but can give you a much sweeter distortion. Just my.002

[Chuck H]

All good info regarding the clipping character of the typical bf/sf topology as it relates to hard rock or metal. But for classic rock, blues, blues rock or jazz the clipping character of those amps has a signature. Lydian seems to like the sound of the amp and needs to control the volume in some way that preserves that sound. In fact, mentioned that distortion pedals aren't offering any joy. To this end it would seem like altering the basic character of the amp for someone elses definition of better overdrive rather than controlling volume would be counter productive.

JM2C

P.S. On the matter of the bf/sf topography for clipping distortion... If you look at a Trainwreck Express or Liverpool schematic (both notorious amps for rock and hard rock distortion) you'll see that the topography is pretty much the same as a bf/sf reverb channel with different component values. So, if you feel like altering one of these Fender amps for better overdrive, it isn't hard to "Wreck" the circuit. I've never tried it but I've seen good results reported.

[Lydian]

I'll admit, attenuator in hand, has made me pretty happy. Going forward, there is always projects on the horizon. I think I'll switch the normal channel to a different tone stack and connect it to the vibrato channel for reverb and the extra gain stage. Then A/B these channels for lead tone when I play heavier. Right now I don't need much, playing with some older guys doing covers like keb mo, James brown, vanmorrison, Taj Mahal. When I get back into some original material, I'll definetaly enjoy the two channel options.

It's a SF and have no plans to ever sell so why not make it custom for my needs. Plus it has made for such a great learning tool.

[Amp Kat]

All good info regarding the clipping character of the typical bf/sf topology as it relates to hard rock or metal. But for classic rock, blues, blues rock or jazz the clipping character of those amps has a signature. Lydian seems to like the sound of the amp and needs to control the volume in some way that preserves that sound. In fact, mentioned that distortion pedals aren't offering any joy. To this end it would seem like altering the basic character of the amp for someone elses definition of better overdrive rather than controlling volume would be counter productive.

JM2C

P.S. On the matter of the bf/sf topography for clipping distortion... If you look at a Trainwreck Express or Liverpool schematic (both notorious amps for rock and hard rock distortion) you'll see that the topography is pretty much the same as a bf/sf reverb channel with different component values. So, if you feel like altering one of these Fender amps for better overdrive, it isn't hard to "Wreck" the circuit. I've never tried it but I've seen good results reported.

That's basically what the KF Komet Amps that my buds at Riverfront Music are building now and are really nice sounding amps. Granted the owner probably works and tweeks more Fenders than anyone I know maybe with the exception of Bruce and they do some pretty unique stuff that KF had passed on to them before he passed. Super cool amps with great tone tstl.

[Chuck H]

Well you came in and showed some concern for how other members might react to modification of a Fender amp. And now:

It's a SF and have no plans to ever sell so why not make it custom for my needs.

You really don't need to qualify it. They command a good market value because they're good amps. Collectability is actually not a huge issue for the sf amps. More for the bf models with the "Fender Electric Instruments" moniker. They have provenance as the last designs made by the original Fender ownership and were only made for about two years. The bf cosmetics continued for another two years made by "Fender Musical Instruments". Not quite as collectable. Then there were the sf amps. Made for thirteen to fifteen years depending on specific model and selling in the tens of thousands. So, supply and demand. I'm hacking a sf Champ way more aggressively than you're mods right now