Amplifier Upkeep

[Rob Gee]

My name is Rob,

I'm a guitar player, a hobbyist builder/tech, and like you all, a valve amplifier enthusiast.

I own a handful of tube amps, but the ones I use most often are my Ampeg B25's. I have one from '68 and one from '74.

I play my main setup 95% of the time, though I really should start playing my other amps more often. This is the Ampeg's setup. 60's Marshall basket weave 4x12 loaded with some unusually warm and well aged Vintage 30's with which the original Greenbacks were replaced. I have that wired in parallel at 4ohms. I leave a dummy cable plugged in to use the 8ohm tap to compensate for the objectionable impedance mismatch. I use a Box-of-Rock overdrive and boost pedal for little kick in the preamp.

Whenever I practice or play a show, I usually play in the same volume range, which is pretty high. Hopefully this doesn't come off ignorantly, haha, I just love playing loud and pushing the amp. Ear plugs are awesome. I run the boost sometimes, but mostly the OD with flat controls 12oclock. I run the amp at about 5 plugged into Channel 1's Normal input. I play heavy music, like hardcore, punk, metal. I'd say I play the amp about 1 or 3 hours a day full throttle (playing frequent shows and practice.)

How much more often would you say I should be routinely checking over the amp? I do my own work, I'm just seeing what anyone's thoughts are on the topic. Aside from that I take care of it very well.

If you'd like to hear the amp, it's on my bands bandcamp. Matahari. It's hands down the meanest amp I've ever played, but always bold and clean. I'd recommend them to anyone for any style of music.

I'd appreciate any thoughts or warnings.

-Rob Gee

[pdf64]

'60's Marshall basket weave 4x12 ... wired in parallel at 4ohms. I leave a dummy cable plugged in to use the 8ohm tap to compensate for the objectionable impedance mismatch'

I don't understand what you mean above - could you explain the set up in more detail?
The schematic shows a switching socket on the speaker output, between 8 and 16 ohm OT taps.
http://www.drtube.com/schematics/ampeg/b25-jp.gif
Pete.

[Rob Gee]

I don't understand what you mean above - could you explain the set up in more detail?
The schematic shows a switching socket on the speaker output, between 8 and 16 ohm OT taps.
http://www.drtube.com/schematics/ampeg/b25-jp.gif
Pete.

My apologies, from looking at the schematic it seems like having just the tip of a 1/4" cable plugged to the "speaker" jack engages the 8ohm output transformer tap. I believe so at least. There is a noticable volume, headroom, and tonal change when using this configuration (the dummy jack, and my cabinet plugged into the "Ext. speaker" jack instead. If there is any information about the setup I can elaborate upon for you, I'd be more than happy. Thank you for your reply pdf64.

-Rob Gee

[Chuck H]

Perhaps the schematics aren't clear. They all show two speakers hardwired to the amp with an extention speaker jack that switches the impedance taps if another cabinet is plugged in, although the b25's I've seen are heads?!? Are there actually two female jacks on the back of the amp??? One main and one extention I'm guessing. Anyway, your correct to plug into the ext jack for switching, but, if the main jack isn't a shorting jack you could just plug straight into the ext jack for the same affect.

Why wire the 4x12 for 4 ohms though? Why not just wire it for 16 ohms and run the cab straight off the head with a proper match?

[rf7]

There are supposed to be 2 outputs on the back of those amps. One is usually an XLR on a pigtail extension cable for the internal speaker, and the other one is on the back panel and it is the external speaker jack. The internal speaker output hits the 16 ohm tap of the OT, while the external speaker jack hits the 8 ohm tap and is in parallel with the internal speaker hookup. Using a 4 ohm speaker cabinet is not correct at all, and could be potentially very bad for your OT. For some reason, Ampeg OT's seem to be tough enough to withstand that mis-match, but I don't recommend it. Rewire your cab for 16 ohms, which is how Marshall wires them, and use the internal speaker pigtail connection. The amp is meant to run one 16 ohm cab, or 2 16 ohm cabs. You can run a single 8 ohm cab on the External Speaker jack also.

[Rob Gee]

Thank you very much for your responses. I do very much understand that the impedance mismatch is taxing my transformer, but I do have a reason for doing so. I have that cab with its 16 ohm V30's wired in parallel by choice. The difference in tonal character is quite stark in contrast to running the same cab matched at 16 ohms. I prefer the 4ohms mismatch. I've never heard anything like my B-25. It's the amplifier I've found "my sound" in, and I like it to sound its best, even with the risk to components. I play best when I know I sound my best, and I sound my best playing on this unorthodox setup. Haha.

[Chuck H]

As far as I can tell from the schem and the info provided your running the amp at a 100% mismatch. Since, as stated, Ampeg transformers are fairly stout I'd say that the biggest injustice is to the power tubes. Running like this they are forced to put out more current than they would with a properly matched cabinet. That said...

I understand your motivation and have done similar things myself. You have my full support. Just know that you will wear out tubes faster than normal. That's part of the answer to your OP. So when the amp sounds like the bottom end is fading, power is lacking or it just generally "feels" like it's working againt you instead of with you, it's probably time to change the power tubes and re bias for the new set.

Also re: the OP, If the power supply filters in your amps haven't been changed out your in for a treat. The amps should perform even better in all respects for the music you play with new filter caps. If this hasn't been done, do it. Those particular caps have a limited lifespan that is more than expired at your amps ages. Be sure to replace ALL the electrolytic caps in the amps including preamp bypass caps and bias supply filters. While your in there use a DMM to check for any "drift" in component values or leaky film caps. Replace anything out of spec (+/- tolerance on the schem). That and a little cleaning of the tube sockets, jacks and pots is standard maintenance for any tube amp.

[Rob Gee]

Thanks Chuck.

Yes these are the original filter caps. I've had the amp like 2 years now, and I've been meaning to replace all of the filter caps and any other components in the signal that drifted. But, my christmas present just happens to be new filter caps and a handful of other components for small future small modifications.

I'll let you know how it ends up with the new caps. I honestly can't wait. Just in time for tour. I'll give them a real good breaking in for 25 days in a row.