'67-'68 Gibson Hawk Amp

[Brad1]

Just found one of these funky late-60's Hawks, great cosmetic condition, at a thrift store, fairly cheap ($30). (Rev/Trem footswitch included). I realize it's a less-desireable Gibson, but for that price...what the heck? It's the tall, skinny one, with a 10" speaker, and front-facing brown panel, Vol, Tone, Trem and Rev knobs.

Three things wrong with it.

Someone stuck a 6x9 coaxial car stereo speaker in it, by securing that on a 1/4" piece of plywood, and securing that to original 10" speaker studs. Fixed, for the moment. Removed, and stuck an old Jensen P10R in, until I decide what else to pop in it.

Reverb doesn't work. Will investigate all that later.

Will replace two-prong cord soon, also.

Most troublesome...this thing is treble to the max! I have heard a couple of YouTube ones, same thing. Others seemed to have tamed it. Anything listenable, you have to turn tone knob down to less than "2". Of course, this also lowers overall volume, and no way to get any more lows in there.

Found a schematic...and these things have a weird configuration. Power switch is in Tone pot, first thing after preamp tube, 12AX7.



Scratching my head on this, trying to think of best way to tame the sizzle, without losing volume. Any ideas to get more "beef" in would be welcome, also. FWIW, it's actually rerally "self-noise quiet", in the hiss or hum thing. Can't hear any noise like that, even maxed just sitting there with a guitar plugged in (guitar volume down), while standing next to it.

Any ideas would be appreciated. I'm just a little confused about that Tone knob power switch, and how to futz with tone, with it there, and the Volume after it.

Thanks,

Brad1

[Brad1]

Wow...dunno what happened to schematic, the "doubling" thing...but there it is.

Brad1

[Enzo]

As to the power switch, they could have put it on anything, volume control being common, but it really doesn't matter. It is an on/off switch, what's confusing?

Start on tone with the speaker, get a less bright speaker. Speakers have the most affect on sound in an amp.

The entire amp[ makes tone, not just the "tone circuit". Have you replaced caps yet? I* might increase the bypass on the input stage, I'd look at the tone shaping right after the volume control, consider larger coupling caps between stages, etc.

[Brad1]

It is an on/off switch, what's confusing?

Start on tone with the speaker, get a less bright speaker. Speakers have the most affect on sound in an amp.

The entire amp[ makes tone, not just the "tone circuit". Have you replaced caps yet? I* might increase the bypass on the input stage, I'd look at the tone shaping right after the volume control, consider larger coupling caps between stages, etc.

Thanks Enzo. Just oddball to me, seeing the tone control where it is. Just got it Friday, so only thing so far is putting SOME kind of actual guitar speaker in, first. And, I have considered a different speaker. A buddy suggested maybe a hempcone Eminence or something. The Jensen is better than the 6x9 coax car speaker, but still way bright.

Your last couple sentences will be considered.

Thanks,

Brad

[drewl]

FYI when you get it up and running try bypassing that filter stage after the volume pot.

Makes it sound alot better and you can always put it back to stock.

[Chuck H]

Gibson liked those T filters. I might put it on a switch and use a push/pull pot if I didn't want to drill a hole. Some guys like the stock tone too so bypassing the T filter with a switch adds versatility and gain.

And +1 to Enzo's suggestion of starting with a speaker change for top end improvement.

[Tom Phillips]

The things I would try to give the amp a little more low end, which will allow you to turn down the tone control without loosing so much volume are:
1) Increase the value of the existing 0.001µF coupling cap on the input preamp stage. Experiment with values up to 0.1µF.
2) Increase the value of the existing 0.02µF cathode bypass cap in the input preamp stage. Experiment with values up to 5uF. You can go higher but just be aware that you will reach the point of diminishing returns by the time you reach 5µF.

Note that the two suggested changes are interactive so, after you play with one and then the other, you may want to go back and adjust the first value etc. You do not need to remove the existing parts to do the test. Just clip or tack solder another cap in parallel with the existing part for now. Your test equipment is your ear so just have fun with it and make the part changes with the amp unplugged from the power source and the caps drained.

Cheers,
Tom

[Brad1]

I think I will start by getting rid of that circuit right after the volume.

Then, try Tom's suggestions.

No matter WHAT speaker goes in there...it won't be enough to tame this thing. So, I'll try to get it this way, first, then mess around with speaker. If I can get it sounding less sizzling on this old Jensen, then a new speaker might be able to help a lot.

For a good representation of what this thing sounds like now, here's a YouTube of someone trying to demo his for Ebay. It's horrendous. It's what this one sounds like with tone anywhere near 2 or above. And, that's with guitar tone rolled back most of the way! Only difference between his and the one I have, is mine has MUCH less hiss and hum...almost non-existent.

https://www.youtube.com/watch?v=yuFlDiotv2g

After looking at some other schems, I can see what confused me. I just couldn't get my head around that goofy Tone Control power switch, for some reason. It all makes sense, now.

Thanks.

Brad

[Chuck H]

I watched the demo and all I can saw is EeW! I'd definitely alter the circuit to more typical guitar amp voicing. But I wouldn't hesitate about the speaker either. Ever hear a guitar amp played into a true full range cabinet? There's all kinds of ugly stuff that gets filtered out by virtue of the poor HF of typical guitar speakers. Especially when an amp is clipping.

[Brad1]

I watched the demo and all I can saw is EeW! I'd definitely alter the circuit to more typical guitar amp voicing. But I wouldn't hesitate about the speaker either. Ever hear a guitar amp played into a true full range cabinet? There's all kinds of ugly stuff that gets filtered out by virtue of the poor HF of typical guitar speakers. Especially when an amp is clipping.

Yeah, Chuck. It's way 'spikey' and harsh. Haven't started on it, yet. Compliling a list of caps and other stuff for a few little projects, to order all at one.

The old Jensen P10R SHOULD be good enough to use to futz with this thing, at first. If I can get the sizzle gone using that speaker, I should be close, and then relatively minor to tweak around a different speaker. Heck, may get it to sound good through it, and save the money NOT buying another speaker. Before I put money into a new speaker, I wanna see if it's even worth it by trying less expensive stuff, first. There are a couple videos on Youtube where they have them sounding pretty decent, AFTER messing with them.

Brad1

[dkevin]

I do not own this era of Gibson Hawk, but I am familiar with Gibson amps of the 60's and their tendency to be harsh or shrill. Most of the 60's amps have the T-filter after the preamp . It is usually a solid orange-red wafer with the letters 10284C inked on it. This is the Sprague network that appears on the schematic. It is written about by a website named Paleoelectronics. It encloses the resistors and capacitors sealed in epoxy. It is my opinion that these amps as newly produced did no exhibit this harsh, metallic tone. If they did, they would never have sold. I suspect that over the years, the values of the components sealed in this epoxy wafer have drifted badly or just failed. This is the cause of the unpleasant tone. If you were to remove the wafer and choose to replace all the components with individual components as the schematic notes, the result may surprise you. However, I usually simply remove the network and make adjustments downstream as required to achieve a quality sound. The good news is, these amps can be had for a song because of the bad tone. The bad news is that chasing other solutions and leaving the #@#@# Sprague network intact is a very costly and frustrating endeavor.

[Brad1]

Most of the 60's amps have the T-filter after the preamp . It is usually a solid orange-red wafer with the letters 10284C inked on it. This is the Sprague network that appears on the schematic. It is written about by a website named Paleoelectronics. It encloses the resistors and capacitors sealed in epoxy. I suspect that over the years, the values of the components sealed in this epoxy wafer have drifted badly or just failed. This is the cause of the unpleasant tone. If you were to remove the wafer and choose to replace all the components with individual components as the schematic notes, the result may surprise you. However, I usually simply remove the network and make adjustments downstream as required to achieve a quality sound.

dkevin,

Nothing like that in there.



Thanks,

Brad1

[dkevin]

That's odd, it shows up on the schematic. Maybe the previous owners already removed it or replaced it with individual components. At any rate, look at the schematic values for the cathode bypass caps on V1 and V2. They seem to be very small. This would emphasize the high frequencies downstream. It might be a good idea to hand-draw a layout labelled with the actual values of the resistors. I usually do this and discover that many of the schematic values have drifted or outright died. Then, with the hand-drawn layout in hand you can decide which components need immediate replacement and which ones will remain. If you are going to modify the original design, let me encourage you to do it one-step-at-a-time so you can measure your progress and determine which changes made the most difference.