4 and 8R output jacks, 4R is switch grounded?

[JazzGuitarGimp]

Hi All,

I am repairing an amp that has a 4-Ohm output jack, tip switched to GND. It also has an 8-Ohm output jack. This means there MUST be something plugged into the 4-Ohm output to be able to use the 8-Ohm output. Questions:

- Have you ever seen this, does it even make sense?
- If there is a 4-Ohm load connected to the 4-Ohm tap, and you also connect an 8-Ohm load to the 8-Ohm tap, doesn't this reflect 1/2 the expected primary impedance?

Thanks
Lou

[Orson Maxwell]

A schematic would be very helpful to reduce confusion.

[martin manning]

Is this a commercially produced amp? Does the 8-ohm jack have any switching function?

[JazzGuitarGimp]

No Schematic, but it's easy to visualize. OT secondary has three wires: Common, 8-Ohm tap (I assume), and 4-Ohm tap (I assume). Common is grounded to chassis. 8-Ohm tap goes to the tip of a 1/4" phone jack labeled "8-Ohms" and this jack has no switch. 4-Ohm tap goes to the tip of 1/4" phone jack labeled "4-Ohms" and its tip switch terminal is grounded.

By the way, I should have mentioned this is a head, not a combo.

I am considering disconnecting the 4-Ohm tip switch terminal from ground, but have this question: Will it be safe to run what I assume is an 8-ohm tap to an 8-ohm speaker without connecting what I assume is the 4-ohm tap to anything; in other words, is it possible I have jumped to an illogical conclusion about the taps, based on the jack labels?

Is there a scenario where the main output tap is indeed 4-ohms, but there is a second tap that can also push an 8-ohm speaker simultaneously with the 4-ohm speaker? If so, what is the actual output impedance of the tap connected to the 8-ohm speaker? This ties in with my question of whether the reflected impedance is half the expected impedance if both taps are used simultaneously.

Thanks for your help, guys. I really appreciate it.
Lou

[martin manning]

It's hard to imagine a commercial product would be wired like that. It must be that the 4-ohm jack is connected to a 4-ohm tap, and plugging an 8-ohm load into the other tap, assuming it is 8-ohms, would indeed reduce the load by half. It makes no sense to use the 8-ohm tap with nothing plugged into the 4-ohm, since the 4-ohm tap is shorted to ground. I would disconnect the CT, put some AC on the primary, and measure the voltages at the two taps (open the shorting contact on the 4-ohm jack with a patch cord) to check the assumption that the 8-ohm jack is connected to an 8-ohm tap. The voltage on the 8-ohm tap should be 1.41x the voltage on the 4-ohm tap. If that checks out, what I would do is put switching jacks in both locations and short the switch contacts together (not to ground) so that either can be used and if no load is connected part of the secondary is shorted.

[JazzGuitarGimp]

Thank you for your suggestions, Martin. I will try these and report back. Question; I am going to disconnect the OT primary CT, remove the output tubes, and inject a 60Hz sinewave into the plates, correct? Can I just use a variac to input the voltage? And is it safe to run the variac up to, say 100V RMS ? And, should I connect a dummy load to the secondary before measuring the output voltage? I realize a variac is not isolated from the line, and understand the precautions in doing this.

By the way, this amp is the Randall Alpha for which I was asking about a schematic a few weeks ago. I saw that Randall is still in business (or are they in business again?) So I sent them an email, asking for a schematic. They told me the Alpha model was very short lived, and they no longer have service documentation for it. IIRC, it has a pair of 8417's and produces about 56 watts into an 8R load from he 8R jack, and about 36 watts into a 4R load from the 4R jack. The difference between 36 and 56 watts into a 4R load is about 1V RMS, so it could have been a bit more than 36 watts. But I saw a bit of crossover distortion into the 4R load that I didn't see into the 8R load. I currently have the tubes biased ar about 75%.

Thanks again, Martin.
Lou

[martin manning]

I am going to disconnect the OT primary CT, remove the output tubes, and inject a 60Hz sinewave into the plates, correct? Can I just use a variac to input the voltage? And is it safe to run the variac up to, say 100V RMS ? And, should I connect a dummy load to the secondary before measuring the output voltage? I realize a variac is not isolated from the line, and understand the precautions in doing this.

Yes connect the Variac across the plate leads, and yes it is a good idea to connect a dummy load to the 4-ohm jack. No need to put a lot of voltage on the primary. I'd run it up until you get 1 VAC across the 4-ohm, and then expect to see something close to 1.414 VAC on the 8-ohm tap.

[JazzGuitarGimp]

Okay, I feel better. With no load on either tap, and with 1.009V on the 4R tap, I've got 1.4258V on the 8R tap, which is very colse to a 1.414:1 ratio. Loading the 4R tap with 4R causes the ratio to increase a bit, which I believe is to be expected. Input voltage was 33.24V, which results in a Zpri of just over 4.4K, which is reasonable for a 50-ish watt amp. Now, to scrounge up a second closed-circuit jack without incurring another $9 in shipping fees.....

Thanks, Martin; I really appreciate you taking the time to help.

Cheers,
Lou

[Phil_S]

Lou,
If it is a Cliff style jack, I'm pretty sure I've got one that you can have. Switchcraft style, can't help you. I'm not sure if PM is working or not these days.
Phil

[Deric]

I've got lots of Switchcraft 12A if that's what you need. Happy to send you one!

[JazzGuitarGimp]

Hi Phil, Deric,

Thanks you both for your kind offers. It is a Switchcraft style jack, and I am pretty sure I have one here. But I may take you up on your offer, Deric, if I can't find one. I have a bigger problem at the moment; this amp has separate chassis' for the preamp and power amp. Most of the work I did was in the power amp chassis - I replaced all the power supply filter caps as well as the screen resistors. Then in the preamp chassis, I removed the death cap and replaced the power cord with a 3-prong. The amp was sounding great after the power amp mods. But after the preamp mods, it sounds like poo.... So, more troubleshooting to do....

Cheers,
Lou

[JazzGuitarGimp]

Problem solved, and it was pilot error. While working on the preamp, I inadvertently hit the standby switch over to standby mode. All this does is bring the power amps' plate voltage down. But the preamp, being all opamps, is always active. When the stanby switch is in standby mode, and you dig really hard into the strings, it makes this splatty distortion sound. Moving the switch to the operate position brought back the tone I was hearing before the death cap and power cord mods. And I found a suitable jack for the speaker output, so thanks again to Phil and Deric. I've got it biased at about 62% (with a whopping B+ of 662V at idle), and it's sounding really good. It's all wrapped up and ready to go back to its owner.

Thanks again for the helping hands...

Cheers,
Lou