Knight Allied Monoblock amp

[angelodp]

Hello, been away for a while. I have an Allied Knight Monoblock in fine condition. It has a great compliment of Mullards. EL37's, 12ax7, GZ34. It plays nicely through the RCA for guitar albeit lower volume. It needs a three prong and a new input for guitar. My questions are twofold.

1. Might any kind soul here have a schematic for this model?

2. What is my best approach to get this to play at volume that is louder for guitar. It has just a the volume knob and no
other controls. It does have a humdinger for the heaters.

Simply add a 1M R to the new input as in a typical amp setup?

Best Ange

[angelodp]

Ok so V1 is an EF86 not a 12ax7. I found a dual version of what I think I have here. I'll post it. So my amp is one half of this schematic otherwise it's very close.

[sluckey]

Radiomuseum has your amp. Several pics, tube list, but no schematic...

https://www.radiomuseum.org/r/allied_ra ... u_793.html

[maxkracht]

Moving the volume pot to after the ef86 is probably a good start, but you would have to change the phase inverter to something AC coupled. Using the 12ax7 for another gain stage then a cathodyne PI would make more sense as a guitar amp than a single gain stage and a DC coupled long tail pair. Not sure how difficult it would be to build a new circuit on the existing pcb.

I think I spot a tiny chief cap, i've had bad luck with those and tend to replace them on sight.

[angelodp]

Hey Max, thanks or that idea. The amp currently is sporting a 5Amp fuse!! As I look at the dual version it shows a 4A fuse. The amp is pulling 1.2A as I run it and revealed by my variac (with sluckey mods). Would my amp being half of the dual version need only a 2A Fuse?

Best A

[trobbins]

Perhaps test how much the volume increases without feedback - as the use of a first stage step network indicates they may be applying a lot of feedback, and certainly more than a guitar amp would typically apply - eg. aim for circa 6dB feedback as a compromise, and that may give you a lot more output.

Imho there is a lot that could 'go wrong' in such an amp, and many such faults could cause collateral damage, which would be sad. So perhaps try and work through how to improve protection from typical faults. Certainly bringing down the mains fuse value to closer to what may be consumed at onset of clipping is a good start, but perhaps have other basic protection in place before stress testing

Even with feedback lowered, the amp may not be very sensitive to common guitar pups, so likely need a pedal in front anyway.

[sluckey]

That monoblock is a ***POWER*** amp! It needs a line level signal input of about 1Vrms to drive the amp to full power. I suggest don't modify anything. Just built a simple one or two tube guitar preamp to plug into the monoblock. A Fender AB763 or Ampeg Gemini II one tube preamp would work nicely with this HiFi power amp.

You probably already have a suitable guitar amp. Just temporarily connect the output of the preamp to the monoblock input to get an idea what the monoblock can do.

[angelodp]

Ok yes will look into that idea Steve. With a simple OD pedal in front this amp is wonderful already. Gotta do the three prong today, get that 'death cap' out. That is a death cap arrangement yes? So far voltages comply with schematic.

Any idea what these T's are - who made them?

I have a proper 1/4" jack now installed in the unused damping factor hole.

BTW did you notice the bias reading arrangement with the 1/4" jack at the power tubes.

Thanks A

[Stevem]

Please understand that since this is a power amp it's looking for a line level input signal which is at minimum .775 volts vac, where as the output from your guitar even with very hot pickups is no more then .250 vac.

[maxkracht]

Might be a good candidate for my LND150 preamp project, though it doesn't look like there's room where the existing rca jack is. Should be fine if you're using the damping hole, though proximity to the output might not be ideal. https://ampgarage.com/forum/viewtopic.php?f=17&t=38039

[sluckey]

Gotta do the three prong today, get that 'death cap' out. That is a death cap arrangement yes? So far voltages comply with schematic.

Any idea what these T's are - who made them?

BTW did you notice the bias reading arrangement with the 1/4" jack at the power tubes.

Those two .01 disc caps are line filters. They help filter out noise on the AC power cord. Once you properly install a 3-wire power cord, those caps are no longer "death caps". I would leave them in place. If they ever fail replace with Class Y caps.

No idea who made those transformers. Potted transformers were popular with high end amplifiers during that time period.

Yes, I noticed the bias test jack. That was another popular circuit during that time period. The jack is wired such that you connect a meter (set to measure dc current) to the jack. This places the meter in series with the cathode to allow you to directly monitor tube cathode current. Did you notice that your amp has one jack and a slide switch to connect to a particular tube? The schematic for the stereo power amp has two jacks and no slide switch. There are probably other small differences between your amp and that stereo schematic. You can use that schematic for testing. It's probably a 99% match to your amp circuit. Don't get hung up on the 1% differences!

[angelodp]

Ah, drat I put in the 3 prong but deleted the y cap. You think it's wise to put that Y cap back in? I did find the actual schematic. Thanks

[sluckey]

I would put it back in.

[angelodp]

Ok, back in like this.... center to ground, each outside leg of cap one to line one to neutral. All good?

Works fine.

[angelodp]

BTW, I tried the NFB R bypassed and it, as expected, is quite a boost in volume. As compared to the boost of a Blues Driver without goosing the Gain knob, it's very similar to my ears. Looking at the pre-amp board max... interesting.

Thank A