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Old 14th May 2012, 01:12 AM   #1
billzoe is offline billzoe  United States
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Join Date: Apr 2012
Default Tube Pre dull into Tube Amp - cause ?

hi all

i have 2 stereo (a Heathkit, and an RCA, both Rebuilt professionally) tube preamps. both are vintage designs c late 1950's early 1960s but have the outputs modded so will drive modern SS amps .

When i try to run them into diff Tube Pwr amps (Heath , RCA Console-All REBUILT professionally), they seem "duller" as if HF is rolling off too much (not the usual Tube Amp roll off this is more severe) -when run into SS Modern Amps-they sound GREAT.

My tech says IF any thing is happening , its just due to "too many tubes" (just 2 preamp tubes + 1 pwr tube per side). This doesnt make sense (to me) since they were (originally-prior to modded output) MADE to go into Tube Pwr amps !

My questions are :

1-is this right? (usually too many tubes to me sounds the opposite-scooped mids and brite hf-too many harmonix)
2-if this is not right, what could it be (impedence issue? due to modern gear interfacing mods on outs)
3-if # 2 , can "I" fix this (i have good solder skills), as i will have to do the work .



Last edited by billzoe; 14th May 2012 at 04:50 AM.
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Old 14th May 2012, 01:14 AM   #2
Join Date: Apr 2012
What does your "output mod" entail?
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Old 14th May 2012, 01:22 AM   #3
billzoe is offline billzoe  United States
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Join Date: Apr 2012
Originally Posted by Alvis View Post
What does your "output mod" entail?
i believe he added a cap to them to "drive modern gear" better (similar to the output cap in the Dynaco PAS mods) on the Heaths' HiZ output. the RCA is Low Z. if anyone needs , i can try to get more details.
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Old 14th May 2012, 10:54 AM   #4
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
Two things:
1. find another 'tech', someone who actually understands electronics
2. post the circuits, with his changes marked

What does "rebuilt professionally" mean? Modded by someone who didn't understand what he was doing?

Dull sound may mean that the amps have too much input capacitance but the solution for that would be to reduce the output impedance of the preamps - needs more than a capacitor to do that. "Too many tubes" is just silly, as you suspect.
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Old 14th May 2012, 08:13 PM   #5
billzoe is offline billzoe  United States
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Join Date: Apr 2012
Thanks... They were re-built by HotGlass audio. But the output cap by my local tech - he's good but is more a SS guy.

Yeah , this makes sense (the input capacitance issue)... i'll see if can get that looked into.

thanks !
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