questions about a Gibson Hawk GA-25RVT

[alange5]

I recently acquired a c. '64 Gibson Hawk (the 2x10 version with 7591's).  It's got new filter caps and a 3-prong cord.  The rest is stock.  Speakers are square magnet Gibson-branded CTS 10's. 


The amp is a bit dull.  It's not very loud and lacks depth.  It just sounds boxy and "blah". 


As I understand it, the Gibson amps from this era used a lot of attenuation and filters to keep things clean.  I'm looking to identify the parts of the circuit that are "sucking tone".  I haven't committed to any modifications, but I'd like to better understand the circuit regardless.


Schematic:
http://music-electronics-forum.com/attachments/40393d1472138853-ga25rvt.gif


A couple things I'm curious about:
-why do the bass controls sit between V1's grids and the volume pots? What's the effect of the bass control here vs. further downstream?
-what's going on around V4A?  Specifically between the C8 .01 coupling cap and V4A's grid?


Any insight is much appreciated.

[Tony Bones]

Everything inside the red loop is a mid-cut filter. This is the deluxe version that Gibson started adding in the 60's, evidently in response to the clean sound that Fender was getting with their blackface amps.

Inside the blue loop is a high-cut filter, of sorts.

I would try completely bypassing the red loop with a wire. You can leave everything in place if you just lift the ground end of R17. The next step would be to remove (or unsolder one end of) C9

[DummyLoad]

Inside the blue loop is a high-cut filter, of sorts.

modified bridged T filter with heavy mids dip around ~500Hz. as you say, lift R17 to disable it.

--pete

[DummyLoad]

bode plot.

[alange5]

Interesting. Thanks for the replies. I'll play around with taking those filters out.


Any comment on the bass control? is that a standard design or would it be better suited in a different spot? There seems to be some interplay with the bass and volume controls.

[PRR]

It's not a bass control. It is a treble reducer.

I don't like its looks. Leaving it on Max Bass should do minimal damage to your signal.

It's not a Fender Gibson Marshall. You can cut it up and make it more-like every other amp in the world, or respect its differences and sell it on to someone who likes the sound of it.

[Tony Bones]

lift R17 to disable it.

I would lift R17 and bypass with a wire.

Any comment on the bass control? is that a standard design or would it be better suited in a different spot? There seems to be some interplay with the bass and volume controls.

I can't comment on how well it works in a guitar amp; you're the only one of us that can. I can look at it, try to understand how it functions, then spill what I think I know:

The crazy network of two 470k resistors, a 0.01u cap, and a 10M pot is really a low pass filter that goes, essentially, from the plate to the grid of V1B. But, since the tube inverts (the signal at the plate is the negative of what's at the grid) it is applying negative feedback around the tube, more in the low frequencies than the highs.

That makes it a low cut filter.

What I can't figure out is why they chose those values (or is the schematic wrong?) The corner frequency is way below the range that the amp can reproduce. 33Hz. That means it presents a constant -6dB/octave slope over the entire useful range. This is not like any tone control that I've seen.

If it has some effect that you can use, then leave it alone. Or make it into something more familiar.

My general approach with amps like this is that if it's really not playable the way it is, then I'll do what I need to do to make it lovable by someone, but no more (I'm currently trying to come to some agreement with a GA-19RVT like this.) Which is why I might disable the bridged T filter and bypass it, but I would leave all the components in place in case some thinks I made a bad decision.

[DummyLoad]

That makes it a low cut filter.

that is incorrect. it is a bridged tee filter - aka, a notch filter.

the values simulate legit at ~520Hz with ~ -35dB dip. it's a second order. has high Q. simulates similar to a fender BF TMB with the mid control at zero.

+1 - sell to someone who appreciates the tone. it's worth more minimally molested.

--pete

[PRR]

> is incorrect. it is a bridged tee

I think Tony is looking at the "Bass" dingus wrapped around the first stage. That's not "bridged". Yes, there is a bridge-Tee further down the line, and we are grateful for your plot.

> 33Hz. That means it presents a constant -6dB/octave slope

It is a mind-puzzle, I agree.

I threw it at the mindless idiot and got these curves. You can hate your bottom octave, or you can hate two bottom octaves. It can't be set to give "full" 82Hz. (Cut any wire on the Bass pot and hear the difference.... but if you want BASS there is much more to ream out.)

Because there is a second bass-cut inside the loop, it approaches 2-pole and nears 12dB/oct. You really won't hear any fundamental on the bottom note with Bass knob set to minimum resistance!

[PRR]

> sounds boxy and "blah".

A speculation:

Played moderate in the shop, it may seem to be missing a lot. Pack a pub with people. Get the bass and drums going good. They will solid-fill the two octaves 60Hz-250Hz, and more. Guitar can't compete down there. Shouldn't waste any of your precious 30 Watts down there. The Bass dingus drops the bass, so you can scream soprano with all 30 Watts.

The ~~600Hz dip is common in guitar amps. I think the strings and pickups center here, maybe to excess. It is a reasonable thing to have. It may not be your thing.

You can try switches to disconnect the Bass network, break the ground on the mid-dip Tee (I don't think it needs a strap-across), and disconnect the Treb which is only top-cut. I think the Treb is there because with the bass cut and the mid-cut, the thing would be shrill. Maybe good for screaming leads, awkward for soft vamping behind a singer.

Or there may be Something Wrong. Have you done voltage checks? Different speaker?

[Tony Bones]

> is incorrect. it is a bridged tee

I think Tony is looking at the "Bass" dingus wrapped around the first stage. That's not "bridged". Yes, there is a bridge-Tee further down the line, and we are grateful for your plot.

Exactly. And I too appreciate your analysis of the bridged T

[DummyLoad]

> is incorrect. it is a bridged tee

I think Tony is looking at the "Bass" dingus wrapped around the first stage. That's not "bridged". Yes, there is a bridge-Tee further down the line, and we are grateful for your plot.

Exactly. And I too appreciate your analysis of the bridged T

deepest apologies tony. i should read things completely, BEFORE posting...   

--pete

[Tony Bones]

No apology needed.  I do the same thing all the time. 

[alange5]

I've yet to experiment with the filters, but I've replaced the cathode caps and plate resistors, and played around with some new tubes, and the amp sounds much, much better than it did. The bass and treble controls are not totally useful, but interesting. The bass control is really only useful set to max. Lowering it produces ice-pick treble. It's a unique sound - possibly useful for clean leads.


The reverb is really gorgeous. Definitely my favorite part of the amp.


The flimsy trem photoresistor legs broke off at the base, so that will need to be replaced. Luckily, I salvaged the original bulb, and it pulses nicely.


I'll report back when I experiment with the filters. If I like the way it sounds, I may put the filters on a switch and stick it in place of either the "monitor" jack, which I have no use for.

[PRR]

> bass control is really only useful set to max. Lowering it produces ice-pick

Non-destructive test. Get three 0.01uFd caps. Tack-solder as shown(across existing caps). This mostly shifts the bass-cut down an octave. Now "full" gives most of your 82Hz ballz, inaudible stock. Less gives less which may be helpful in a band setting (don't put your power where the bassist is working).

[alange5]

Before I mess with the filters, I'm having a heck of a time trying to track down some intermittent static/crackling sounds.  I've swapped tubes, cleaned and tightened sockets, and voltages are on-par with the schematic.  The crackling is present with or without a guitar plugged in, and it's a consistent volume independent of any controls.  Chopstick poking doesn't seem to make any difference.  Removing V4 makes it go away.  Any ideas where to look?

[alange5]

Still having trouble finding the source of the crackle.  I replaced all the plate resistors in the preamp, replaced 4v's cathode resistor and cap, tried disconnecting the LDR, tried bypassing the filter between V2A and V4A... no change.  I used some alligator clips on V4 to try a new socket, but the crackle persists.  In the power section, it's got new filter caps, new dropping resistors, new screen resistors.  Removing V4 is the only thing that stops the crackle.  Is it possible the PI transformer is the problem?

[punkykatt]

Do you have Doug`s listening amp tool? If not build one. I had that model amp in the shop for the same crackling problem. Ended up being the driver transformer for the power tubes.

[alange5]

Ended up being the driver transformer for the power tubes.

Do you remember what you used as a replacement?  I think the Hammond 124A should work....

[tubeswell]

Are all the input jack tip switches making proper contact when they close?

[alange5]

Are all the input jack tip switches making proper contact when they close?

Yup. Crackle is there whether they are open or closed. Based on all my tests, I'm looking at the interstage transformer. I'll wire up a listening amp to confirm.

[punkykatt]

Mercury Magnetics has them ( TF-1001-D )

[shooter]

I've used this one on 2 GA5's


http://www.triodeelectronics.com/unintrui.html

[alange5]

I ended up using a cheap 1:3 interstage transformer from AES and the crackle is gone (I believe the original is 1:2.5).  Thanks again for all the help.