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Discussion in 'Amp Tech Center' started by Si G X, Feb 6, 2020.
Yes, HiWatt used it in their 1964 103. Very effective as well for tone bending.
I'll watch with interest. My first amp was a Selmer in the late 60s. As soon as it warmed up you got overdrive whether you wanted it or not!
Jon Snell, I have to ask....what is tone bending? Is that another way of saying tone adjustment?
Well, thinking about it, I guess it's not that different to the presence control in a Marshall (or HiWatt etc.) except that rather than feeding back to the bias circuit of the PI it's going back to the single input pentode and is the only tone control available instead of complementing a TMB stack.
Staring at endless 5W SE schematics will do that, I guess - you forget how the big boys do it
Tone bending adjusts the frequency of the negative feedback so effectively increasing the gain by a few dB at the resonant frequency set by P2 and C5.
HiWatt improved on this as did the more 'modern' Dynatron Music Centre' with their contour control.
This is how I modify the negative feedback in my Snell 66 All Valve Head;
The NFB comes in through R27 and adjusts the phase as required.
Thanks, Jon. I just call that a tone adjustment since all tone control happens to be frequency specific gain control of one sort or another. Next time I hear/read tone bending, I’ll know what someone means by that...hopefully. I have never heard it called that before.
Hear is a reference to what I have heard called tone benders.....
Basically they are fuzz pedals with a tone control. Wonderful technology from the 60s and still sought after.
Thanks for the link, it brought back many memories of pedals that didn't work properly. The countless hours I spent fixing them.
Great fum in the 60s.
I am in the middle of producing a valve compressor with extra features. Only because a band in my area cannot purchase the sound they want and they know I can make them what they want. I will post the outcome when it is perfected with the types of sub-miniature valves shall be using.
Si G X, what tools do you have for servicing? Are you keen to do electrical testing if it doesn't cost much to gather a few devices and take your time to do testing?
The simplest servicing would at least use a light-bulb style limiter, but in this case I'd recommend a megohmmeter, a variac, a test valve rectifier made from solid-state diodes, and some measurement tools.
If I had your amp, I'd summarise a restoration path that measures and inspects all parts as much as practical, including confirming that mains AC side insulation is ok (eg. at 1kVDC megger level) and 250VDC on secondary side wiring and 1kV on output transformer primary. Then solder in a B+ load and without valves and using an ss rectifier, bring up the secondary voltage and confirm that parts and wiring are not faulty (testing for coupling cap leakage) up to the working voltage of the caps. Then repeat with signal valves in and a load for speaker, and confirm that bias levels increase appropriately as B+ is ramped up. If that is ok then swap back to valve rectifier and ramp back up again watching the levels. Then move to noise/hum testing using a speaker, but without input. Then use a tone input and check the feedback level and output response, preferably with a spectrum analyser to check if there is any inadvertent peaking or hum. Then double check that the mains fuse rating isn't too high, and add an appropriate CT fuse to guard against collatoral damage when the EZ80 finally dies.
If you do get too bogged down with this nice little amplifier, take the chassis out of the box and disconnect the speaker, (yellow wire to the upper lug), remove the valves, wrap them up and send the amp to me, I will refurbish it for you for just the cost of the spares and postage back. This amplifier is too important to lose.
If you are OK to proceed, be careful and enjoy the task. I wish you every success.
That is so good of you to offer, Jon.
Si G X: that's a fabulously generous offer almost too good to refuse, though I'd understand if you prefer to delve in yourself.
Sound clips please guys once this thing has a clean bill of health.
Thanks Jon... that's very generous and would be amazing, I'll be removing the chassis at the weekend anyway and I'll gently clean some of the dust away too. I think I would be foolish to say no to that.
Maybe this some kind of good Karma coming back for all my friends guitars and basses I've spent hours repairing and setting up for nothing?
Seriously, thank you... Jon.
That would mean you'll have to take pictures and keep this thread going though.
Sound clips for sure. Absolutely ....
yeah, no ... I'm more in the camp of "I have a soldering iron and multi-meter and know how to you use it, and I know how to drain caps so I don't electrocute myself"
I think I need to admit that this needs checking by someone else. I can replace a couple of caps, burnt out resistors, blown diode or whatever, but that's about it.
I can take photos but I will leave the sound bit to you.
My email is firstname.lastname@example.org
Thanks Jon, I'll pop you an email over in a minute, so we have contact.
A few more pictures before it heads off to Jon next week.
Removed the knobs, it's a shame one of the metal arrow tops is missing, while I was taking them off the other one fell off too, which is probably a good thing as that 57 year old glue isn't up to much at all. I'll glue that back on with a tiny dab of super glue later.... I guess my search for a replacement for the other one starts now.
Time to unsolder (or should that be desolder?) the wires from the speaker. A little cloth just in case and a little heat and the wires were free.
Looks like a spider met his doom in here at some point, not much left but a few legs.
That dirt and filth on the window surround came off really easy with a slightly damp cotton bud, it looks clean and shiny now!
I found this interesting... a little piece of the lighter coloured covering cloth on the inside. For a reason? Making minor repairs?
Here's a few shots on the underside....
As well as the spider legs, I also found this poor woodlouse, just a husk of his former self.
There's some markings here on the chassis in what looks like pencil maybe.
... and a Sel(mer)10 stamp here.
The amp sits on these little white plastic feet...
Sadly I have one of the feet missing.... I'm pretty sure I could make one of these out of a bit of white nylon rod/dowel or something like that, there not much to them. Interesting to see what looks like a rivet head under the foot and not a screw.
I'm really looking forward to hearing what Jon has to say about it when it arrives, the valves are wrapped up and boxed too, they look good and I'm hoping they work good too.
Don't use Superglue! Use Evostik.
Better still try here;
Looks like just the brass inserts on the bottom row.
I ended up using a little dab of Araldite expoxy, seems to have done the job. I gave both knobs a light clean too, they look really nice now.
Thanks, I'll get one... the knob looks alright now with the old brown glue removed, but it would be nice to have the brass trim.
The amp is boxed up ready to send tomorrow Jon.