Matching output impedances: how do?

[bobshbob1]

Hey all,

I posted here a little while back about getting a more crisp, less muddy sound out of my Fender 140 head. I tried swapping tubes and removing some high frequency roll-off caps to not much avail. Then I tried using the head for bass. And it sounds amazing, like it was originally tuned for bass more so than guitar, and it's just as loud as the 700-watt solid-state monster I had been using.

So my issue is this: The bass cab I'm using is a 4-ohm 2x15. The amp has a "speaker" output and an "extension speaker" output, both of which are labelled at 8 ohms minimum load. I know how with tube amps, it's generally safer to use a speaker with a lower impedance than the amp is expecting than one with a higher impedance, so I feel comfortable hooking this 4-ohm cab to the 8-ohm output on the amp.

However, given the choice I'd obviously rather have all the impedances match, I'm just not sure how to do it. Since the speakers are each 8 ohms, wired in parallel to give 4, I suppose I could just connect each one to the head separately. Looking at the schematic, though, it looks like both OT secondary leads go to the "extension speaker" jack first, and plugging into that changes which tap is used. It makes sense in my head that you would plug one 8-ohm speaker into the "speaker" output and use only the 8-ohm tap, plug a second 8-ohm speaker into the "extension speaker" output and use only the 4-ohm tap for both, or plug one 4-ohm speaker into the "extension speaker" output and use only the 4-ohm tap. Is this how it works or am I missing something?

Schematic for those interested:
http://www.stratopastor.org.uk/strat...-140-schem.pdf

[pdf64]

If a jack plug is inserted into the extension speaker jack socket, the socket's built-in internal switch should change the active OT secondary tap from 8 to 4 ohms.
Maybe use two speaker leads between amp and cab (good idea with all that power)?
Or just plug a dummy jack plug into the extension speaker socket.
A word of warning though, those switching jacks can get a bit dodgy, and so it may be more robust to hard wire the green/yellow wire to the speaker socket 'hot' terminal.
Those amps put the output tubes under a lot of stress; they were designed around an STR487 (special tube request) 6L6GC variant, which was more akin to a 7581A 35 watt military tube. Modern 6L6 may not last for long, so best to use the right impedance load; also the bias supply voltage can be rather low for some modern 6L6 types, changing to a full wave bridge rectifier can help to raise it a few volts and may be sufficient. Or a voltage doubler circuit can be used, but the key thing is to check the output tube dissipation; suggest 1 ohm resistors are inserted into each output tube cathode ground return, so that the dissipation of each ouput tube can be ascertained.
Pete

[Tom Phillips]

Since the speakers are each 8 ohms, wired in parallel to give 4, I suppose I could just connect each one to the head separately.

Yes. You could do that.

...Looking at the schematic, though, it looks like both OT secondary leads go to the "extension speaker" jack first, and plugging into that changes which tap is used. It makes sense in my head that you would plug one 8-ohm speaker into the "speaker" output and use only the 8-ohm tap, plug a second 8-ohm speaker into the "extension speaker" output and use only the 4-ohm tap for both...

That’s all correct.

...or plug one 4-ohm speaker into the "extension speaker" output and use only the 4-ohm tap. Is this how it works or am I missing something?

That won’t work because the “speaker” jack has a built in shorting switch. It shorts the OT secondary to ground in an attempt to protect the amp in case someone tries to play it without a speaker connected. Because of that design you must always plug into the “speaker” jack first.

You could rewire the amp or put a dummy plug into the “speaker” jack with your 4Ω speaker load connected to the "extension speaker" jack. However, you would need to be careful to remember that you are using the amp in a non-standard configuration. Personally, I wouldn’t have a problem doing that if it was my amp but I wouldn’t recommend it to others for fear that they would make a mistake at some time, damage their amp and blame me.

Tom

[bobshbob1]

Ahh, apparently my schematic-reading skills are not the best. I didn't catch the main speaker jack shorting to ground. I guess I'll go with a dummy plug in the extension speaker jack; I'm the only one who uses the amp but it lives in a practice space where it's technically available for others to use when I'm not there, so I feel like a temporary change would be safer than permanent rewiring.

Also, regarding pdf64's comment, are there any current production 7581A or similar type tubes out there? (Would modern KT66s work?) A quick check on google only turned up fairly expensive NOS ones.