Amp Tech Thread / Ask a tech Q

[hisheroisjon]

My V-4 keeps blowing power tubes, generally taking out smaller components with it, my tech has it biased for jj 6550's, but I think that's what's causing the problem. Would 7581's be a viable alternative? I like the clean headroom of the 6550 but I don't think my amp likes them.

[Mr. Foxen]

Full going over to find actuual problem, rather than throw more valves at it. Including if you cab is the problem.

[hisheroisjon]

Oh of course, I brought it to my tech this morning and while we checked it out then, he's going to over it more thoroughly. My cabs shouldn't be an issue,  they're sunn/fender reissue 4x12s and  have no problems. From what it looks like the JJ's and sovteks I've tried don't handle the voltage of the v-4 well and fail easily.

[dunwichamps]

it was designed to use 7027a, but there is no true modern equivalent for them in terms of voltage specs.

[FullCustom]

it was designed to use 7027a, but there is no true modern equivalent for them in terms of voltage specs.

I just put a set of JJ-7027A tubes in a V-4 on Friday. They worked just fine. I've had 5881, 6L6GC, 6550, and modern 7027A tubes all work fine in a V-4 with just a little bit of modification. I'm sure that I could get other tubes such as KT77 or EL34 to work as well.

[dunwichamps]

it was designed to use 7027a, but there is no true modern equivalent for them in terms of voltage specs.

I just put a set of JJ-7027A tubes in a V-4 on Friday. They worked just fine. I've had 5881, 6L6GC, 6550, and modern 7027A tubes all work fine in a V-4 with just a little bit of modification. I'm sure that I could get other tubes such as KT77 or EL34 to work as well.

they will function but it puts stress on the screens. The screen voltage is overspec and so its more liable to burn out the power tubes. Dropping the screen voltage down and bumping up each screen resistor to 1k 5W will help to prevent this problem.

[hisheroisjon]

I got the amp back about a week ago but have been to busy to take it to the space. Apparently there was a diode (I forgot where he said exactly, bias supply maybe?) That was letting over a large amount of voltage, so that could have been the culprit. It also now has a quad of Sylvania 6l6gcs that were rebranded for mesa that I got for a cool 100 bucks.

[sheikyerbouti]

i recently bought a brand new fender deluxe reverb reissue, and after playing it a few times it´s developed this nasty glassy rattle that occurs everytime i play an E (on any string or octave).i´ve been looking through a lot of threads on other message boards on this and haven´t found a solution yet , or someone who has the exact same problem for that matter.
any advice would be appreciated

[VOLVO)))]

Dead speaker or loose speaker.

[Pissy]

Get a friend to poke around on the thing until you find the offending physical issue. All while you play a Scott Pilgrim E note.

[FullCustom]

Did you buy it new or used?

[sheikyerbouti]

i bought it brand new
the rattle also pops up at very low volume

[The Bandit]

Take that shit back yo.

[FullCustom]

i bought it brand new
the rattle also pops up at very low volume

If it's less than 30 days old just send it back to the store you got it at. If it's less than 90 days old even tubes are still under warranty. Fender's warranty and service are really good.

[liquidsmoke]

Does the type of magnet in a humbucking pickup have any bearing on how much it hums/feeds back? Alnico II, V, ceramic?

[beerrhino]

I have an old univox amp that has a blown output transformer.  It has 2, 4 and 8 ohm taps.  Is it possible to replace the OT with a transformer with 4, 8 and 16 ohm taps?

[dunwichamps]

yes, just need to find the "right" primary for the power tube setup you have and adjust which tap the feedback is coming off of.

[morgantician]

Sorry if this has been touched on before. I did a search but saw nothing...

I'm going to retube my OR120. I've had a tech do the bias for me in the past but this time I'm going to do it. As you may know, these have no standby. How does this affect the boasing process?  The step-by-steps I've found heavily noted what to be done in and out of standby, so how will this change things?

Thanks!

[dunwichamps]

in order to bias the amp, it needs to be completely on as you need to measure plate voltage and cathode current

[aowron]

Since it looks like the preamp tubes are almost worn out in my amp (they have gotten microphonic once; just squaling making the amp unplayable), I'm thinking of replacing them, and at the same time resolder one of the input jacks. This does though lead me to the question: what is required to actually drain the caps? I've seen quite a lot of different answers regarding this, although I found this today: http://www.youtube.com/watch?v=sw9KnSFy3Fs, where it looks like the tubes basically will discharge just by running the amp hot, and then turning it off.

Also, it seems like my amp was not as loud as usual last time it was used; my 330watt hybrid bass amp could easily keep up with it (and the amp this is regarding (the guitar amp) is a Laney GH50L connected to a 4x12). Could this be due to crappy preamp tubes or could it mean that I also need to replace the power amp tubes?

A bit of history on the tubes in the amp: When I bought it, the power amp tubes had recently been replaced (Mullards; although newly made (thus made in the Sovtek factory in Saratov), but were apparently expensive as hell), while the preamp tubes were some old sovteks from the previous owner's JCM2000, thus, this leads me to believe that the EL34:s should have some time left since the amp only has been used in a Jam Room for about a year, while the preamp tubes probably are worn out.

[moose23]

Anyone familiar with early 70s Ampeg 2x15s? I see one going for a fairly decent price here but curious if they're worth it or should I just stick to my plan of building myself a Sunn215B replica. Will be used on bass with a Sound City 120 MK4

[FullCustom]

in order to bias the amp, it needs to be completely on as you need to measure plate voltage and cathode current

I think what he is referring to a method of measuring bias current by placing your meter across the open standby switch. Some old-heads would do Fender amps that way. It does work, I've tried it.

[liquidsmoke]

How much distorting can Eminence Swamp Thangs take before they begin to become damaged? At practice/gig volume really weird shit(strange pulsing that sounds bad) starts happening if I have too much low dialed in and I suspect that no damage will occur before that point but I'm not completely sure. I want as much thick low end/low mids as possible even if it's ridiculous for guitar.

[Jake]

A bass player friend ran her Ampeg B2R with a 4ohm and 8ohm cab together. The amp's min load is 4ohm, so she roached something. What ususally takes the shit in that situation and is there anything that can be done?

[VOLVO)))]

Any number of things can pop in that situation... They will be apparent when you open it...

The swamp thangs shouldn't be distorted or fucking up at alll, they're ridiculously resilient