Gibson Hawk questions

[drewl]

Got one of the late 60's models, two 6BQ5's, reverb, trem.
Anyone know how much power they usually put out?
I'm getting like 10 - 12 watts.
The amp has like a fizzy type of distortion, even with different speakers.
It's kind of "screetchy" with the trem turned up, looking at the the second stage after the tone and volume pots, turning up the treble turns the sine wave into a wave with two peaks on the top.

That's after the twin "T" filter.
Bypassing the T filter I get a nice clean signal to the output tubes, but it's driving the amp too hard, clipping the output signal a bit.
Sounds great, but maybe a bit too distorted.

I feel like I'm trying to polish a turd here. It's just a low budget beginner amp, are they supposed to sound good?



It's the first, schem. don't know why both got linked.

[dwmorrin]

Is it just me, or is that kind of an odd thing to have a notch filter in the middle of the main signal path?
I'm no filter expert, but after a bit of googling and calculating it looks like the notch is supposed to be around 482Hz. Is this serving some noble purpose here, or just unnecessary tonal surgery? Was Gibson trying to counter a 482Hz boost somewhere else... or did they just think that notch sounded cool?

I've no Hawk to measure the output for you, but I seem to recall that this series of Gibson amps are generally underwhelming.
If you're getting better results without the filter, I say cut it out and put a resistor divider in its place to put the right amount of signal into the next stage.

[olddawg]

I had a Skylark with 6bq5s that I loved. I finally put a JBL in it and gave it to a really good harp player I know who used it for years before it was stolen. Harp players like Hawks too. Are you sure it has fresh electrolytics and tubes? 18 watt Marshall's and Vox AC 15s use those tubes and sound great.

[pdf64]

Check out the tsc Download notch filters centered around a few hundred Hz is what tone stacks do.
So it's a budget amp with a similar scooped mid tone to a more sophisticated stage amp.
Plenty of more recent designs have the same idea.

10-12W seems reasonable, even 15W is optimistic from a pair of cathode bias EL84 fed from a tube rectifier.
Fizzy distortion can be a characteristic of them too, though typically, the wimpy Gibson OT probably doesn't help.
Pete

[drewl]

I found some clips on you tube and it seems to normal, the fizz when turned up.
I went through every inch and everything is working normally
like I said, bypassing the T filter makes it sound better, but it breaks up too early.
I even tried a Marshall 18watt type tone circuit with the same results.

[dwmorrin]

Just curious if you bypassed the filter with a short, or did you try and bring the level down with a resistor?

Looking backwards from the "Voltage Amp" grid, there's 2x 220k (440k) series resistance for the dry signal, and 470k series resistance for the reverb signal. If you don't put back that series resistance when you bypass, I can see how it would overdrive the power amp too quickly.

[drewl]

Even with the reverb tank disconnected, the rest of the reverb circuit is still there, plus it was "raspy" with the tank connected.

Originally I just jumpered a .02 across the 500pf to bypass the T filter, yeah I could use a divider to knock the signal down, but turning down the treble yeilds the same result so I put it back to stock.

It's weired because with a signal generator the signal looks pretty good on a scope even at different frequencies.

[olddawg]

There are many more knowledgeable people on this site. But the first thing I would do is make sure the filament voltage is not being pulled down somewhere and maybe use a diode rectifier to bring the B+ up a bit. Also, all el84s are not created equal.