5E5 Build - trying to add a cascaded gain channel

[Csquare4]

I recently completed an amp project designed around a 5E5 Pro schematic which turned out to sound very good. Since I only used one input, I decided to put a switch that would jumper internally the two triodes in V1 so that they were in parallel as to have a little more breakup. That worked well...sounded great. The only real mod from the standard 5E5 was to add a Lar-Mar Post PI Master Volume.

As an additional mod, I wanted to try and get a little more into high gain territory if possible by cascading the triodes to see if I could make it work. I have a buddy that is asking me to build him an amp and he wants some grit for when he is in a "heavy-ish" mood, so I thought I would would see what I could do with this build.

I installed a 3-position rotary switch in an effort to keep the "Single (3)" and "Jumpered (2)" modes, but to be able to add the "Gainy (1)" mode. So here is what the schematic looks like:



Conceptually, it still works well up until a point. The initial 2 modes still work perfectly, as expected since nothing really changes there, but when put the switch into mode 1 it gets really raucous. Which I kind of suspected as well. I am trying to maybe figure out a way to reign it in a bit, at anything above about 8-9 o'clock on the "gain/volume" knob, it just gets out of control and starts dropping out. I suspect I am overdriving the PI to some extent, or getting severe blocking distortion. Also, it looks like maybe the bias point shifts pretty dramatically in the 2nd triode when the cascade is engaged. So maybe there isn't a simple way to do this without changing substantially the rest of the circuit, but I just wanted to throw this out there to you guys and see if anyone has some suggestions.

I measured the voltages in the three switch modes, so that may give some insight as to what's going on.

Mode 3 (single in) and Mode 2 (internally jumpered) are pretty consistent. Voltages stay pretty much the same in either switch position.

B+ 380V

V1 Pin 1 107V
V1 Pin 6 107V
V1 Pin 3 1.3V
V1 Pin 7 1.8V

V2 Pin 1 254V
V2 Pin 6 313V
V2 Pin 3 2.1V
V2 Pin 7 68.1V

With the Switch in position 1, cascading the triodes of V1, things don't look too bad, granted the bias point shifts up the curve.

B+ 358V (this mode switches in a 100k plate resistor to triode 2)

V1 Pin 1 140V
V1 Pin 6 165V
V1 Pin 3 1.75V
V1 Pin 7 3.75V

V2 Pin 1 239V
V2 Pin 6 294V
V2 Pin 3 1.96V
V2 Pin 7 64V

So the only thing that looks out of whack is the bias voltage on the second triode of V1 (and possibly the first triode as well). Could that be the cause of the dropping out, it could be hard clipping of the signal due to the high bias voltage...not sure. I am just looking to see if there is maybe a reasonable way to drop that bias down into range by maybe going with a lower plate resistor (56k maybe?) on the 2nd triode when this mode is switched in. That would not effect the other two modes.

Any thoughts on whether this might work or if I am just chasing my tail. Thanks in advance.

Craig

[pdf64]

That 2nd triode needs a grid leak to ground - it's essential.
A grid stopper (and ideally a 1M trim pot for a pre gain control) on that grid circuit may be very beneficial also, otherwise it will likely oscillate once the grid leak is sorted out.
Also the functionality of the tone &gain/vol controls would be improved if arranged like the Harvard http://www.webphix.com/schematic%20h...5f10_schem.pdf

[Csquare4]

That 2nd triode needs a grid leak to ground - it's essential.
A grid stopper (and ideally a 1M trim pot for a pre gain control) on that grid circuit may be very beneficial also, otherwise it will likely oscillate once the grid leak is sorted out.
Also the functionality of the tone &gain/vol controls would be improved if arranged like the Harvard http://www.webphix.com/schematic%20h...5f10_schem.pdf

PDF -
Thanks for the input, I was planning on putting a grid stopper in series with the .022 coupling cap feeding the 2nd triode, that was my next thing. I will try a 470k ala Marshall to see if that helps. Also, I debated quite a bit about the grid leak, I couldn't figure out how to make it work with the switching so that it was out of the circuit completely when the other two modes were engaged. I will try a 1Mohm from maybe the input of the 2nd triode to ground, at least it will be taken out of the circuit when the switch is in the 3rd position (single in). I would love to put a 1M pot in there as well, I just don't have the real estate. But, you definitely are pointing me in the right direction I think, I was trying to simplify it too much.

As for the tone stack (Vol/Tone pots), I will change those out like you suggest and see how it works. They are actually pretty responsive now but that is an easy swap.

Thanks again for the help.
Craig

[pdf64]

'I couldn't figure out how to make it work with the switching so that it was out of the circuit completely when the other two modes were engaged'

OK, have the 0.022uF cap feed a 1M trimpot, then take the output of the trimpot to position 1 of the switch (ie where the cap currently goes to).
If no room for a trimpot, then a couple of 470k in series to ground.
Even with that, a 10k grid stopper may save some grief.

[Csquare4]

'I couldn't figure out how to make it work with the switching so that it was out of the circuit completely when the other two modes were engaged'

OK, have the 0.022uF cap feed a 1M trimpot, then take the output of the trimpot to position 1 of the switch (ie where the cap currently goes to).
If no room for a trimpot, then a couple of 470k in series to ground.
Even with that, a 10k grid stopper may save some grief.

This is what I was thinking after the last comment -


I have already started working on this one. I am hoping with the 1M grid leak, it should be mostly transparent. The 470k grid stopper seems to be common in Marshalls with cascaded gain stages, so that I why I chose that. It may actually tame some of the gain as well. If this doesn't get me anywhere I will backtrack and try your new suggestions.

[Csquare4]

That last configuration has me almost there. I agree, I need to figure out some way to put a pot in front of the 2nd triode to tame it a bit. As it is, this thing is extremely loud unless the volume and master are turned way down. But it has a pretty intense distortion going on, which is what I was going for. The low notes are flubby when going through this stage, so I need to work on tightening those up as well.

But the key is I think this may actually work now. Much more stable. Thanks for the input.

Craig

[Csquare4]

PDF -

Again, thanks for the pointers. After having my wife threaten me with a frying pan last night, I decided that I really needed to get the "gainy-ness" under control...it was basically a binary amp when the cascaded pre-amp was engaged. Either "almost no volume" or "freaking ear-bleed loud". I added a 1M pot, to the input of the second triode when this mode is switched in. This makes it much more manageable and allows you to dial in the amount of "distortion" coming out of the pre-amp. Actually works great, between about 8-10 o'clock on this pot, I am getting a nice saturated tone, akin to a tube screamer, that just builds in thickness. By dialing in the VOL/GAIN and the MASTER, I am really getting some great tones at lower volumes. By cranking these two controls, it gets loud. I am liking this build now.

I still need to take care of tightening up the low end, I think I am saturating the OT when the gain is high. Sounds ok with Humbuckers on the neck, but the bridge gets pretty muddy down low. Debating on lowering the bypass cap value on V1 (or putting them on a switch) to see if that helps. I just don't want to change the characteristics of the other two modes. All in all, I am pretty pleased. This is one Tweed design that now has some serious bite.

Here is the latest schematic for the preamp if anyone is interested.

[pdf64]

Craig, well done.
You could reduce the low end flub by reducing the 0.022uF cap that feeds the 1M dist pot, by a factor of ~10, try 2n2F. Also the cathode bypass on that 2nd triode could be reduced to ~1uF. Or leave it unbypassed, as you have plenty of gain already.
The 470k 'grid stopper' is a high value and will be rolling off treble, which could make the low end seem excessive in comparison; you could try bringing it down to 100k or less. However, best to put an additional 10k grid stopper with one leg soldered on the tube socket terminal.
Pete.

[Csquare4]

Craig, well done.
You could reduce the low end flub by reducing the 0.022uF cap that feeds the 1M dist pot, by a factor of ~10, try 2n2F. Also the cathode bypass on that 2nd triode could be reduced to ~1uF. Or leave it unbypassed, as you have plenty of gain already.
The 470k 'grid stopper' is a high value and will be rolling off treble, which could make the low end seem excessive in comparison; you could try bringing it down to 100k or less. However, best to put an additional 10k grid stopper with one leg soldered on the tube socket terminal.
Pete.

I will try those suggestions and see what works best. Thanks again for the pointers Pete.

[Csquare4]

I think I am pretty much finished for now. I did as suggested above to try and get the gain under control, worked like a champ. I changed the inter-triode coupling cap to a 0.0022uF and put an ON-OFF-ON mini-toggle switch for the bypass capacitors on the 2nd triode, using the 25uF and a 2.2uF with the center position being open. This gives a few options for tightening up the bass frequencies so that it doesn't get flubby. I also reduced the 470k grid stopper to a 100k. I didn't put a 10k on the tube terminal only because it would require me having to really dismantle a lot of circuit to get to it at this point...so I will reserve that until I figure I absolutely need it. Right now, it seems ok.

The amp really retains it's tweedy roots when the switch is in the 2nd and 3rd positions, but when it is in the 1st hi-gain position it really becomes a scorcher. There is some low level hum in this stage, but for the most part, the amp is really quiet compared to some commercial like powered tube amps.

The amp is loud and playing through an old Ampeg 4x12 loaded with Celestion V30's and H30's it is very nice. The mode 2 is really creamy, it is pretty easy to dial in some nice Clapton tones. Also, the amp is pretty touch sensitive and in this mode, rolling back the volume on the guitar, it cleans up nicely. In the high-gain (mode 1) it somewhat loses itself in low, fast riffs, but I suspect that is as much to do with the 5U4GB rectifier tube. I have a copper-cap I may try if it doesn't raise the B+ too much. Actually, I am quite pleased. Here is a copy of the preamp schematic if anyone is interested.



The power output section is essentially a 5E5 Pro with a Lar-Mar PPIMV, running 6L6's cathode-biased right at 70% dissipation using a 330ohm resistor.

My next project will be to see if I can figure out how to control the switch with relays so I can build a footswitch. Also, now that I think I have a pretty good grasp of how to do this, I have an old Traynor YBA-1 that could use some work, so I may actually try some of this on one of the channels in it.

Again, thanks for the help Pete.

Craig

[pdf64]

The power output section is ... running 6L6's cathode-biased right at 70% dissipation using a 330ohm resistor

Craig, the 'set bias for 70% plate dissipation at no-signal' guideline isn't intended for cathode biased outputs; for guitar applications, in which the power amp is likely/intended to be overdriven, as the value of cathode resistor is increased, the power output becomes restricted and potential for crossover distortion increases. If viewed on a scope, the negative impacts of this on the output waveform are apparant, and can be correlated with the consequence on tone, becoming 'thinned out'.
See Biasing
scroll down about 2/3 of the page
'Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow. '

Also, I just noticed the 0.1uF coupling caps - they may be causing blocking distortion (eg flub in the V2a or power tubes) under overdrive. Consider reducing the one feeding the vol/gain to 10nF or 4n7F. Depending on how you've arranged the master vol, the ones on the output of the phase splitter could be reduced also; as an example, for 220k grid leak resistors, 0.047uF is plenty for a full low end on a bass amp, never mind guitar.
The rule of thumb I read somewhere is that for full audio low end, Cap (in uF) x Reistance (in k ohms) =~ 10
Hence the 0.047uF 220k combo; if you want to tighten up the low end, bring it down to 5, 2 or even 1.
Hope that helps - Pete

[Csquare4]

Craig, the 'set bias for 70% plate dissipation at no-signal' guideline isn't intended for cathode biased outputs; for guitar applications, in which the power amp is likely/intended to be overdriven, as the value of cathode resistor is increased, the power output becomes restricted and potential for crossover distortion increases. If viewed on a scope, the negative impacts of this on the output waveform are apparant, and can be correlated with the consequence on tone, becoming 'thinned out'.
See Biasing
scroll down about 2/3 of the page
'Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow. '

Pete -
Thanks for hanging in there with me on this thread. Your insight has been immensely helpful. I have used the Aiken site for knowledge for several years now and am familiar with this page regarding biasing. I always wondered about whether I should set the cathode bias amps closer to full power dissipation, but when I go higher, it just sounds too rich as if I am right on the edge of redplating the output tubes. I originally went with a 250ohm cathode resistor, but even though it sounded super (And LOUD!!), I was right at 90-95% and was just worried about getting too hot. I agree it does sound thinner, but I just don't want to be in a situation where my tubes start melting if I am at a jam or my son is using the amp in one of his performances. Maybe I will just go back and take it on faith

It did sound good for sure. Even biased low, it is still a very loud amp.

Also, I just noticed the 0.1uF coupling caps - they may be causing blocking distortion (eg flub in the V2a or power tubes) under overdrive. Consider reducing the one feeding the vol/gain to 10nF or 4n7F. Depending on how you've arranged the master vol, the ones on the output of the phase splitter could be reduced also; as an example, for 220k grid leak resistors, 0.047uF is plenty for a full low end on a bass amp, never mind guitar.
The rule of thumb I read somewhere is that for full audio low end, Cap (in uF) x Reistance (in k ohms) =~ 10
Hence the 0.047uF 220k combo; if you want to tighten up the low end, bring it down to 5, 2 or even 1.
Hope that helps - Pete

I debated about the 0.1uF coupling caps, but I was really trying to stick with the Fender 5E5 schematic. I get that those are pretty huge and probably a bit responsible for the loose bass, so I was going to check to see if I could tighten it up a little there as well. Right now it sounds pretty decent, however, those coupling caps are an easy swap, so I should plug some new values in and see if it tightens it up any more. I don't want to loose too much of that old school tone though. But easy enough to explore.

Thanks again for the help and the wonderful insight.

Craig