Questions from a new member

[blave]

Hi all,

Fascinating forums here at the Garage - very enjoyable reading. By way of introduction, I am a lefty player living in San Jose, CA, USA. My current posse consists of: Matchless Skyliner, Fuchs-modded Fender Blues Deluxe, Framus Ruby Riot, Mesa Maverick 1x12 (which I will probably sell shortly). I went through a period of Amp Acquisition Syndrome back in the early 2000s but I am on meds now, so other than jonesing for an 18w Marshall clone I am doing fine, one day at a time....

(For the Dumblephiles here - you have probably seen most or all of them, but two or three years ago I collected all of the Dumble photos I could find on the web (less some that RobL asked me to remove a while back) and posted them on my photo site:

http://blave.smugmug.com/Music

You will see that there are other amp-related things there too.)

Anyhoo, I have some questions that I thought I'd toss at the sage wizards here...

1. why do (most?) amps with tube rectifiers have standby switches? It is my understanding that the rectifier will sort of self-regulate the voltage/current "hit" on the other tubes by way of its own warm-up time.

2. I have seen arguments that you shouldn't put an amp (either SS or tube recto) on standby once it's been turned on - e.g. at a 15 minute band break - it's better to just turn it off. Opinions? I would think the thermal cycling on the tubes that results from turning the amp off completely would shorten their life.

3. I just bought a used AC30-derived Framus amp - it's EL84-based (of course) with what I believe to be cathode bias. (BTW I loooove this amp!) Can I pull two of the four power tubes for reduced power? I used to know the answer on this, back in my AAS-suffering days, but it escapes me now. I *think* the answer is "sorta, but it will change the tone since the impedance matching between the output tubes and the tranny will become non-optimal."

Thanks in advance for your input. I will probably have more questions as they occur to me...

cheers,

Dave B.

[mlp-mx6]

I'll take a crack at answering your questions.

1. why do (most?) amps with tube rectifiers have standby switches? It is my understanding that the rectifier will sort of self-regulate the voltage/current "hit" on the other tubes by way of its own warm-up time.

Not all rectifier tubes are all that "slow." Tubes like the 5U4 can begin conducting VERY quickly. Indirectly-heated rectifiers, like the GZ34, are the ones that have a nice delay (and voltage ramp-up) to make it easier on the filters. Regardless, I would expect that many of them have a standby switch due to convention.

2. I have seen arguments that you shouldn't put an amp (either SS or tube recto) on standby once it's been turned on - e.g. at a 15 minute band break - it's better to just turn it off. Opinions? I would think the thermal cycling on the tubes that results from turning the amp off completely would shorten their life.

This one is not "decided." Most guitar amps are around 500V or less - which is definitely high voltage, but not high in the realm of tubes. 1000V is the *beginning* of HV for tubes. So many folks do not feel that a standby switch is of much value period. Idling is not bad for an amp - certainly not for 15-30 minutes on break. Thermal cycling is not all that good for tubes - and the most important thing in a normal-running amp is to try to keep it cool. A slow-moving fan works wonders!

3. I just bought a used AC30-derived Framus amp - it's EL84-based (of course) with what I believe to be cathode bias. (BTW I loooove this amp!) Can I pull two of the four power tubes for reduced power? I used to know the answer on this, back in my AAS-suffering days, but it escapes me now. I *think* the answer is "sorta, but it will change the tone since the impedance matching between the output tubes and the tranny will become non-optimal."

You will throw off two things by removing the tubes. You will have an output impedance mismatch, but this is not all that critical. You can compensate by switching the impedance tap on the output, if that is available. However, if this is relatively true to AC-30 architecture and is cathode biased, your cathodes will also be at half of the voltage they're supposed to have, and the plate voltage may well be higher - not a good combo. Probably will sacrifice tube life in this instance, not to mention the likely significant change in tone and response.