Power transformer clipping?!

[jimboyogi]

Thanks for the extra info MWJB and Steve,

MWJB, I implemented the master volume mod you suggested and it sounds much better, much less treble attenuation at lower settings. Now the amp really shows up it's poor low freq. response, so I'm going to put a proper OT in, otherwise I'll never know. The only problem with the master volume now is that most of the useful range of operation is in the first 1/3 of the pot rotation. The pot is a Log (Audio) taper, am I right in thinking that this is a better option than Linear taper to get the most useful range?

Another question.
The guy I am making this for has requested that the Hi and Lo inputs can be foot switchable by using an A/B switch box. I currently have the inputs set-up by the standard Fender method, where I lose the voltage divider function on the Lo input when a plug is inserted into both of the inputs.
My jacks are Cliff mono insulated, with break switching when a plug is inserted.
Is there any easy way to set-up the inputs so that I can still get a 6dB attenuation on the Lo input when both inputs are being used?

Cheers

James

[jimboyogi]

Hi All,

I have posted a brief build thread for this amp here

OffsetGuitars.com • View topic - Building the RhoneyTone

Thanks to all of you for your help so far.

Cheers

James

[Bruce / Mission Amps]

...Another question.
The guy I am making this for has requested that the Hi and Lo inputs can be foot switchable by using an A/B switch box. I currently have the inputs set-up by the standard Fender method, where I lose the voltage divider function on the Lo input when a plug is inserted into both of the inputs.
My jacks are Cliff mono insulated, with break switching when a plug is inserted.
Is there any easy way to set-up the inputs so that I can still get a 6dB attenuation on the Lo input when both inputs are being used?

Cheers

James

You might have a misunderstanding of what is happening there with the two jacks and the three resistors.
I would say no because the two 68K resistors are actually mixing the inputs when you use both jacks at the same time.
And, when doing that the there is no 6db attenuation from one to the other.
I'm not 100% sure here but I don't think you can have it both ways... because, plugged into the amp, you are "mechanically" utilizing the two switches on the jacks... using an external foot pedal means the two switches in amp jacks are always open (when two phone plugs are inserted into the amp's jacks).
You could just rewire both jacks so they each have two different resistor values to simulate a large input signal loss from one to the other.

[J M Fahey]

There's a simple solution for this.
What your friend *really* needs is a footswitchable 6 dB (or 10 dB if he so wishes) attenuator.
Piece of cake: build it (two or three resistors and a dpst switch in a little box) , which will now provide the same functionality as plugging your guitar either into A or B jacks, but with half the hassle.

[jimboyogi]

Thanks for the replies,

J M Fahey, that is a great idea! If I used a pot then the attenuation would be adjustable as well. Only needs 2 guitar leads for the setup instead of 3 also.

Maybe set-up the pot as a rheostat to ground? 1 meg?

Cheers

James

[J M Fahey]

Yes, that's it, wire the pot as a regular guitar volume control, input jack to hot and ground, output jack to a switch that selects either hot or wiper.
The guitar should be always full on, of course, because this is an all passive chain, you don't want to lose more useful signal than necessary.
That 1M audio pot won't affect your sound much. You could solder a 100pF cap, hot to wiper. to lessen treble loss.
Of course that cap should be a Solen Auricon Super Audiophile Institute approved matched dielectric cap, with electrolytic gold leads and white whale oil and calf skin papyrus , to have an "enhanced spaciousness sound stage" and "mellow but not brittle" silky mids.
Or you can just use a 100pF ceramic cap.

[jimboyogi]

Hi All,

Just wanted to give an update on this amp.

I ended up re-wiring the master volume as a potentiometer, as I was worried about the effect on V1b ac load line when it was wired as a variable resistor. I put a bright cap across the pot to try, and then found that I didn't need it, as I couldn't hear any treble attenuation when the pot was turned down.

As I did all this it dawned on me that the amp was sounding 'muddy' from too much mid frequency. The 5F2-A tone control passes pretty much all mids, and as a consequence humbuckers were sounding terrible through this amp. So I replaced the tone control with a mid-cut control, from a design in Merlin B's book. This made a huge difference to the versatility of the amp, and allowed more apparent volume with less distortion as the mids were attenuated.

Then I replaced the line matching transformer with an Allen Amps SE one (TO11C). When it arrived I thought that it had no chance of changing the tone, as it was virtually the same size and weight as the line matching one it was to replace. But HOLY COW it did change the tone, giving a much bigger bass response, and extra clarity. This has made a big improvement to the apparent headroom of the amp, now through an efficient speaker (EV15L) this is LOUD, even before distortion.

Now this amp is a master volume, and the idea is to have a bit of distortion/clipping on tap from V1b. Because the mid-cut control attenuates all guitar frequencies to some degree, if you scoop the mid a bit then the signal getting to V1b is attenuated, and I found that I wasn't getting enough signal there to give the distortion that I wanted, especially with NFB on. I tried increasing V1a plate load resistor, but this didn't give enough extra gain.
So I ended up taking ThermionicScott's advice from from much earlier in this thread (thanks ThermionicScott ), and using a BF Champ style V1b cathode, with a partially bypassed cathode. This has done the trick perfectly, now the amp can go from very clean scooped BF style to very thick dirty lead tones.

I'm still only getting ~ 6.4 WRMS output just before onset of distortion, but I'm now happy with that

Thanks everyone!

[J M Fahey]

Then I replaced the line matching transformer with an Allen Amps SE one (TO11C). When it arrived I thought that it had no chance of changing the tone, as it was virtually the same size and weight as the line matching one it was to replace. But HOLY COW it did change the tone, giving a much bigger bass response, and extra clarity. This has made a big improvement to the apparent headroom of the amp,

Well, now you know . Congratulations.