Need a bit more volume on the clean channel

[JoeK]

Hi still building my Hotcat clone http://www.webphix.com/schematic%20h..._hotcat_30.pdf I've got both channels operating fine tone wise but the clean channel seems too quiet to my ears. It was just one triode as per the spec driving the phase inverter and cathode biased EL34's. Wired like that it puts out about 16VAC across the 8 ohm load with a 200mv input so about 30 watts?. Nice and clean but I can put guitar and amp on 10 and sit in front of the speaker without any discomfort. Next I wanted to add a reverb circuit to the clean and this is operating well but loads down the clean output even more. I'm not a designer or E/E so I admit I'm Ok at building proven circuits rather than creating them I do have a spare triode so I've tried to add another low gain stage after the clean-reverb mix but I'm just creating a loud noisy channel out of a quiet and clean one. Here's the circuit as is without the extra triode http://i188.photobucket.com/albums/z...vSchematic.jpg
Are there any examples of other amps with a topology like this? I know the Fender Deluxe reverb etc as I've tried to slot in the same circuit. Any advice much appreciated. Joe.

[jimboyogi]

Hi JoeK,

First up, on the Hotcat schematic I think that there is a big error on the Gain channel, the first gain stage has no bias for the cathode, the cathode is just grounded.

Now to your question. There are a few options here with the spare triode.

Option 1 is to use 2 triodes paralleled for your Clean first gain stage, should give you a bit more gain and lower noise. Will also decrease your output impedance, and therefore lead to less loading of the output by your tone control/vol control/reverb takeoff. I don't follow your reverb blend via the .01 cap. What is stopping the AC reverb signal from passing through the other .01 cap and back into the reverb driver, creating a feedback loop?

Instead of blending the reverb back in this way, why not instead insert it to one PI input, but combine the 2 dry signal pre-amps via ~ 220K isolation resistors into the other PI input?

Option 2 is to use the spare triode to cascade a second gain stage in the clean channel. You can control the gain by playing around with options like - leaving the cathode un-bypassed, splitting the plate load resistor and taking the output from the tap, or even using a lower gain valve like a 12AY7.
Another advantage of using cascaded stages here is that the clean and gain channels will now be in phase, so that if they both go into the one PI input, you can "jump" or "link" them for even more gain/harmonics.

Good luck and enjoy!

[pdf64]

It may be beneficial to investigate why the 1 stage clean channel doesn't have sufficient gain. The normal channel of an AC30 can be driven into overdrive from a regular guitar; that's a similar arrangement, though el84 has more gain than el34.
So, check what voltage gain you're getting out of each stage (pre, phase splitter, power) to see if any seem lower than expected. Pete

[kg]

Hi JoeK,

First up, on the Hotcat schematic I think that there is a big error on the Gain channel, the first gain stage has no bias for the cathode, the cathode is just grounded.

probably run in grid leak bias. it should actually work.

[JoeK]

Yes it's grid leak bias on the gain channel but the resistors are swapped before the first grid. Well I've gone back to square one and found if I lose the reverb feed and return I get a decent voltage across the amp. I thought I had done this before but I think I left the reverb feed connected to the driver tube. So with a 200mv 1Khz feed I can read 8vac from the gain stage through to the PI grid and about 50vac on the grids of the power tubes. This puts out 18vac across the dummy load but I'm using a Fluke 73 mk1 which isn't true rms but I'm hoping is in the ballpark. Sounds like a respectable blast from the speaker now.
If anyone could advise a better division of signal between the PI feed and reverb driver tube I'd appreciate it. Or perhaps I have a fault that's dragging my signal down? Cheers Joe.

[jimboyogi]

My mistake about the grid leak bias on the gain channel V1a. I'm interested to know what the grid voltage is at quiescense? I though grid leak bias used much larger grid leak values than 1M.

Re. the reverb return, here again is my suggestion from above -

Instead of blending the reverb back in this way, why not instead insert it to one PI input, but combine the 2 dry signal pre-amps via ~ 220K isolation resistors into the other PI input?

I have done this before and it works well to my ears!

[Chuck H]

Just a thought... With the PI biased a little colder than many other amps as well as having a very large tail resistor compared to other EL34 amps I would guess that you'll have an easier time increasing gain at the PI than the first triode. This will affect both channels though. IMHO a "clean" channel should have at least enough gain to get the power tubes to clip a little even with parallel single coil pickups in order to use all the amps power. But perhaps this amp doesn't do that. With one triode and a cold biased PI with a large tail feeding a pair of big bottles I'm not surprised at all by your findings. You could try changing the PI bias resistor to 820 and the tail resistor to 15k. If the change in tone on the dirty channel isn't objectionable it may give you what you want from the clean channel.

The stock amp may have had enough gain. Your reverb coupling circuit taps a bit of gain off the triode and as you can see by the 220k plate resistor the designer was already trying to get as much gain from the one triode as possible.

[pdf64]

The gains seem a bit on the low side, though I wonder if the PI and power amp are overdriving a little?
If you turn the vol down, what is the Vac on the 1st stage plate with the 200mV in? Then increase the volume to put 1Vac to the PI input and see what the PI and power amp output is.
The amp seems to be loosley based on an AC30 normal channel, especially the PI. Pete.

[Chuck H]

The amp seems to be loosley based on an AC30 normal channel, especially the PI. Pete.

Yeh, but an AC30 uses el84's which are much easier to drive. Reducing the output of the first triode by taxing it with the reverb circuit, combined with the lower output of the PI may be what's keeping the amp from reaching full output power. Since the triode is already doing all it can I think experimenting with increased PI output might be worth a try.

Looking at some calculators I see that changing the tail resistor won't make a significant difference. But changing the plate loads to 150k will add about 6V of swing... Not much.

You might do well to rewire the reverb to drive with one triode and make the other a recovery stage.

And FWIW 30W from a pair of cathode biased el34's at 380Vp doesn't sound unreasonable. What kind of pickup are you using? Does a loud humbucker get you into a little clipping on the clean channel?

[JoeK]

My mistake about the grid leak bias on the gain channel V1a. I'm interested to know what the grid voltage is at quiescense? I though grid leak bias used much larger grid leak values than 1M.

Re. the reverb return, here again is my suggestion from above -

Instead of blending the reverb back in this way, why not instead insert it to one PI input, but combine the 2 dry signal pre-amps via ~ 220K isolation resistors into the other PI input?

I have done this before and it works well to my ears!

Hi thanks for the ideas, should stimulate some thought patterns here I hope. The grid voltage on the gain stage is something I hadn't noted down before. To clarify the schematic shows a 5.1meg gnd off the input jack and 1meg off the grid to ground. They are reversed to the working amp. The grid has the 5.1meg to cathode and ground. The grid at zero input is -0.6VDC the plate voltage is 120VDC with my wall voltage at 250VAC being a bit high.

[JoeK]

The gains seem a bit on the low side, though I wonder if the PI and power amp are overdriving a little?
If you turn the vol down, what is the Vac on the 1st stage plate with the 200mV in? Then increase the volume to put 1Vac to the PI input and see what the PI and power amp output is.
The amp seems to be loosley based on an AC30 normal channel, especially the PI. Pete.

Ok still have the amp with just the clean channel (no reverb send or return connected) on one side of the PI, Gain channel into other side of PI. With the 200mv feed and the volume on zero I read 12.5vac at the volume pot input (straight after the plate coupling cap)
With the volume up to a 1VAC at the PI input I get 10VAC at the PI output and 9vac at the dummy load. EL34 plates to cathode are 392/394VDC Cathodes are 29 and 30VDC. I have changed the cathodes to 400ohms each as the original 270 was just too hot and cooked a tube during clean channel sound testing. With Weber's bias calculator that gives 28watts per tube at 71ma per tube which is spot on with my bias meter testing. Full volume current draw was about 100ma plus per tube with the 200mv input.

[JoeK]

Yeh, but an AC30 uses el84's which are much easier to drive. Reducing the output of the first triode by taxing it with the reverb circuit, combined with the lower output of the PI may be what's keeping the amp from reaching full output power. Since the triode is already doing all it can I think experimenting with increased PI output might be worth a try.

Looking at some calculators I see that changing the tail resistor won't make a significant difference. But changing the plate loads to 150k will add about 6V of swing... Not much.

You might do well to rewire the reverb to drive with one triode and make the other a recovery stage.

And FWIW 30W from a pair of cathode biased el34's at 380Vp doesn't sound unreasonable. What kind of pickup are you using? Does a loud humbucker get you into a little clipping on the clean channel?

I've been using a Les Paul with Bare Knuckle PAF pickups for testing plus I've got a G&L strat with Seymour Duncan single coils. The LP is obviously a lot louder and it was just getting a clip before. Now the volume is getting quite thick with the reverb gone so it looks like I need to rethink how to implement it. I think the actual amp has a subtle reverb compared to a surf variety from other's comments. Any candidates for a single triode send and recovery with low impact on the clean signal? The original amp photos do show a dual triode send but I can't see anything much after that.

[Chuck H]

Looking at your drawing you seem to have an unused triode in the amp. Why not revamp the reverb circuit a bit and use the extra triode as a mixer? Then you can keep the dual triode driver and likely increase both clean channel gain and reverb level if desired.

[JoeK]

Looking at your drawing you seem to have an unused triode in the amp. Why not revamp the reverb circuit a bit and use the extra triode as a mixer? Then you can keep the dual triode driver and likely increase both clean channel gain and reverb level if desired.

Yes I tried that as per the start of the post:
"I do have a spare triode so I've tried to add another low gain stage after the clean-reverb mix but I'm just creating a loud noisy channel out of a quiet and clean one"
I tried copying the fender AA763 recovery setup but it sounded bad or non existent. Possibly because I'm only using one triode for the initial gain? where the usual Fender setup is input-gainstage-tonestack-gainstage- then reverb. I'm guessing that since I'm losing all the clean signal when I connect the reverb send, and it has that 4 volts on the rev driver cathode instead of 8 volts, I've got a problem around that point?

[JoeK]

Just thought I'd follow up with some positive news in that the clean seems to be kicking out as much volume as it could before the reverb was added. I've ended up using the dual triode driver and single triode recovery but I added some more resistance to the signal mix. I have 150K from clean signal to PI and 360K from reverb return to PI. The reverb is not Fender surf by a long way but it doesn't have that sound on the original amp either. More experiments with the spare gain stage to come.