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Post subject: Ultralinear Twin tube heat
Posted: Wed Mar 30, 2011 12:59 pm
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I inherited a 77 135w ul twin reverb from my dad, who played it since he bought it new. He doesn't play anymore so he finally handed it over.

The first time I played it it sounded awful.
I changed the tubes, and it got a little better, changed the speakers, a little better still, put in a pot to control the nfb, even better.

Still was just too bright sounding, not like I remember it sounding when I was a kid. :?
I changed the inner two tubes to run in "triode" mode, wow what a difference. I put a switch in to change it betweed two ul/triode and all four triode mode. sounds so much better now.

Problem is that the tubes appear to be getting so much hotter now than they did before. They were really not that hot before, even after a four hour gig, now they're super hot after a couple of hours. Is this normal, is it OK?

On another note, the great sound from the twin that I remember was actually from my dad's blackface 65 pro. :oops:


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Post subject: Re: Ultralinear Twin tube heat
Posted: Wed Mar 30, 2011 1:09 pm
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What power tubes and what's the idle bias set at?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Ultralinear Twin tube heat
Posted: Wed Mar 30, 2011 1:51 pm
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The power tubes are some of the Fender GrooveTubes 6L6GE from when they first came out.

How do I check the Idle bias, can I do it with a standard DMM?
I haven't done anything to the bias circuit, its just got the "Output Tubes Matching" pot in it, and that is basically sitting in the middle of travel.


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Post subject: Re: Ultralinear Twin tube heat
Posted: Wed Mar 30, 2011 2:09 pm
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This is the recommended bias probe......

https://ssl.eurotubes.com/cart/index.ph ... ory_id=109

The ultra-linear amps do not have a traditional variable-bias circuit -- basically, the output tubes are matched (in pairs) using the chassis's "balance" control, similar to the adjustment available in the AB165 Bassman. If you've bought a quartet of balanced 6L6CG's there may not be a lot to be done. However, the UL amps are brutal on output tubes, with plate voltages in excess of 490 VDC. Only the hardiest of bottles should be used in these models. Suitable new glass would be the Sov-Teks, EHX, and JJ's. Winged "C" tubes should be avoided as well as any NOS vintage 5881 types.

Your overheating problem likely is a result of running only two 6L6's and I do not recommend the practice.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Ultralinear Twin tube heat
Posted: Wed Mar 30, 2011 2:51 pm
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I'm running all four power tubes, but now they're wired triode instead of ul.
I just checked and it appears that the speakers were wired in series :shock:
I'll wire them up for the correct load and check it out.
I measured the plate voltage at 500vdc.
Brutal indeed.


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Post subject: Re: Ultralinear Twin tube heat
Posted: Wed Mar 30, 2011 3:01 pm
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Yeah, the 4Ω load is preferred for your amp but the UL Twin's all had a dual-secondary OT with self-switching speaker jacks. They'll run anything between 2Ω and 8Ω safely......16Ω is a bit of a stretch though.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Ultralinear Twin tube heat
Posted: Wed Mar 30, 2011 3:09 pm
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I'll get a bias probe and check the reading.
The speakers were in the ext jack, for the 8Ω tap but were in series for a 16Ω load.

To be fair though, before switching it to triode I never turned the amp up much past 2, with the center tubes in triode, I had it up to three, so I'm sure they were working harder, especially with the impedance mismatch. The amp does sound so much better now though.
:)
just hope it don't eat too many tubes. Matched quartets aint cheap.


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Post subject: Re: Ultralinear Twin tube heat
Posted: Wed Mar 30, 2011 3:27 pm
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My '78 UL TR ran for 23 years on the original glass (Fender-branded Westinghouse). I had it re-capped and re-tubed in 2001 with RCA NOS blackplates. It's doing fine thus far.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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