More gain from Allied Knight?

[dmartn149]

I've converted an Allied Knight PA amp to guitar use and it sounds pretty good, but not very loud unless I run my tube screamer in front. It has two identical channels with two gain stages each, and I was thinking I might be able to combine those into one channel with 4 gain stages, and two or three volume controls.
I'm new to amp mods so I wanted to lay out my plan to the forum to see if you all think it is the right way to do it, or not.
I was thinking I could take the output from "mic 1 control" and send it directly into "mic 2" input, then take the output from pin 2 of the second 6sc7 and send it into pin 4, maybe going through the "phono control" pot first. Should I delete the .05 cap right after the "mic 2" input, and add a .05 cap after pin 2 of the second 6sc7? Is that the right way to do this? Should it be done at all. Pins 2 and 5 of v2 are linked together. what do I do about that?
Thanks, Dan

[Tom Phillips]

Four stages of pre-amp is probably not necessary. If your tubes and components are in good condition you should be able to get sufficient gain by re-configuring your first two stages. Look at the Fender 5C5 Pro circuit as an example. Here is a link to the schematic http://webphix.com/schematic%20heave..._5c5_schem.pdf.
Tom

[dmartn149]

Thanks Tom.
That 5c5 doesn't look substantially different from the Knight except that I aready have an extra gain stage. I wonder if I have a bad preamp tube.

[Chuck H]

I don't see an extra gain stage on the schematic.?. Could you possibly paint on that schem image to show what circuit changes you've made?

[dmartn149]

I meant that I have an extra gain stage compaired to the 5c5 pro. Each channel on the pro gets 1/2 of a 6sc7 then on to the PI, right?
The only changes I've made to the Knight is replace the obsolete in/out jacks with 1/4" jacks, replace power cable, add fuse, remove "death cap"
Will a bad preamp tube still pass a signal? Is there any way to test them? (I don't have any more 6sc7s on hand)

[Chuck H]

It's not likely that both your preamp tubes would be bad in the same way. What happens when you use the other "channel"? If both channels behave the same, try removing the preamp tube from one of them and stick it in the PI socket. This will cause a small bias difference in the preamp, but it may reveal if one of your tubes is bad.

[dmartn149]

Both channels sound about the same. I think that both channels use both tubes. Channel 1 uses 1/2 of v1 then 1/2 of v2. Channel 2 does the same.

[Chuck H]

Ah... Funky. I would have thought the designer would have the insight to use one tube for each channel. That way, if one channel goes down because of a bad tube the other one still works. Just the same, swap one of the preamp tubes for the PI tube and see if anything changes. If it does then you'll need to get at least one 'known good' preamp tube for swapping in the sockets.

If there is no change then I would guess that all the preamp tubes are working correctly (if one were bad, or had a bad triode, it should change the nature of the problem when swapping tube positions). In this case you should take voltage readings on all tube pins and post the results. Remember that the filaments are AC and measured from one end of the winding to the other. All DC readings should be with the black probe on the chassis and the amp at idle.

[dmartn149]

I just tried swapping v2 with the PI and there was no change. When I measure the filaments end to end,(tomorrow) would that be pins 7 & 8?
Also I just noticed that the volume pots on this amp are 500k not the usual 1meg. Would that have an effect on volume?

[Chuck H]

1M rather than 500k doesn't amount to much. Less than 1dB of difference in actual gain. I don't think the circuit, as it is designed to work, is the problem. series triodes, low pass filter tone stack, a paraphase inverter, a pair of 6l6's at 415Vp and no negative feedback should have about the same volume and gain as a 50W Marshall. So I'm guessing that something is "broken". Broken could mean the bias is out of whack, a cold solder joint, etc. That is to say, if the circuit were working correctly you wouldn't be having this problem. So rather than trying to find fault in the design, we need to find out why it's not working as it is designed to. What about the speaker cabinet? Have you tried the same guitar through another amp? What kind of guitar are you using?

Voltage reading are really the next step. Read preamp filaments with the meter set to AC between pins 7 and 8. Read 6l6 filaments between pins 2 and 7. All other pins should be tested with the meter set for DC (high volts on all plates and a lower volt range setting on other pins). Black probe on the chassis and amp at idle.

Just write the tube/pin#/voltage. As in Mic2 first triode pin1 = 0V, Mic2 first triode pin2 = 100V, etc. don't neglect any negative voltage readings (though I don't expect any).

Do this for all pins, all tubes.

[dmartn149]

HAHA! Thanks for not letting me rewire my amp.
I re-cleaned the pots, re-heated all my solder joints, and tried another guitar. It sounds much better, pretty darned loud, though not enough to run me out of the room. I think the biggest problem is that other guitar. I just got it last week, on ebay, and didn't realize how low the output was. I just have a few things to do to it then it's done.
Thanks again