Tube help

rocknroll1

rocknroll1

Well-Known Member
I have a VTL TL5.5 preamp early model with phone stage. It has 2 12ax7's and 4 12at7's. I am getting a loud hiss from all sources, it does not increase with volume knob. It's not so loud that I hear over the music but loud enough to hear without anything playing. I think I have narrowed it down to old tubes and I would like to try some new tubes in it. The gain very high to the point that I can only use the 1st 1/3 of volume knob. Does anyone have any recommendations? If they lowered the gain it would be a plus but really more concerned in quit and warm.
 
couple of points

your unit needs to be diagnosed as to where the noise exists.
1, use phono input, use tape output to another amp.
2. use non-phono inputs, tape output to another amp
3. using another source, like CDP, use its output into tape monitor in

no noise in #1 eliminates phono section
no noise in #2 means noise in line stage
#3 noise confirms #2

next up, download schematic and inspect the tubes, usually numbered V1 to v6
unless there's a tube rectifier which you did not specify then either the signal
tubes are shifted by 1 OR there's a V7. tube rectifier is first or line in line

then look carefully at schematic, usually one channel is displayed. and if
in one channel all V-numbered tubes are used then it means the other triode
in these 12xx7 tubes are used for in both channels. if not, for example even numbered
tubes in one channel and odd numbered channels in the other (whether displayed
or not), then note such.

the next steps are to swap out old tubes with new in one channel to see if the noise
level is reduced by comparing this channel with the other. if noise is reduced
then your theory is correct. otherwise, not.

it is unlikely the 12ax7s, with their high gain, is used in the line stage. usually
in the phono section for exactly high gain to handle the RIAA filters and
resultant impact of gain.

however, if for some reason, 12ax7s are in the line stage, you can use 5751s
as a direct electrical replace. these have about half the theoretical max gain
of the 12ax7 cousins.

there are no direct tubes to lower the gain of the line stage if they only use
12at7s. you will need to go in and change the design if you feel the gain
is too high. I suspect it is only because today's modern devices have
far higher outputs, thus your struggle with the volume knob. I suggest
that if this is the case, and you believe the gin is too high, then go into the
source devices feeding this preamp and check the back for output gain
and lower if there is such a control. it would be a crime to mess with
the preamp. although when the spilt window corvette coupe of 1962 (only)
was introduced, there were folks that removed the split window thereby
destroying the value of the 1963 corvette coupe.

another way to lower gain is to use the tape monitor to pass all signals
(meaning the tape monitor is always engaged) to an external box with
super high quality film deposition resistors (per channel) to reduce
the signal. this assumes your preamp has a tape monitor circuit.

if the new tubes do not reduce the hiss, then the next step is to
replace the resistors. first off, ALL must be measured to ensure
they are within margins of the original design. you will need to
find period-correct AND identical to the original vendor in specific
product line, value, wattage, voltage handling, size, etc.
if you don't use original resistors, and for example, you
use modern metal films (at 1W or better) then the sound
will change. as well as resale value.

there are more changes but we'll leave them out for now.

the key is how much effort needs to overcome the hiss
annoyance.
 
Bob said:
couple of points

your unit needs to be diagnosed as to where the noise exists.
1, use phono input, use tape output to another amp.
2. use non-phono inputs, tape output to another amp
3. using another source, like CDP, use its output into tape monitor in

no noise in #1 eliminates phono section
no noise in #2 means noise in line stage
#3 noise confirms #2

next up, download schematic and inspect the tubes, usually numbered V1 to v6
unless there's a tube rectifier which you did not specify then either the signal
tubes are shifted by 1 OR there's a V7. tube rectifier is first or line in line

then look carefully at schematic, usually one channel is displayed. and if
in one channel all V-numbered tubes are used then it means the other triode
in these 12xx7 tubes are used for in both channels. if not, for example even numbered
tubes in one channel and odd numbered channels in the other (whether displayed
or not), then note such.

the next steps are to swap out old tubes with new in one channel to see if the noise
level is reduced by comparing this channel with the other. if noise is reduced
then your theory is correct. otherwise, not.

it is unlikely the 12ax7s, with their high gain, is used in the line stage. usually
in the phono section for exactly high gain to handle the RIAA filters and
resultant impact of gain.

however, if for some reason, 12ax7s are in the line stage, you can use 5751s
as a direct electrical replace. these have about half the theoretical max gain
of the 12ax7 cousins.

there are no direct tubes to lower the gain of the line stage if they only use
12at7s. you will need to go in and change the design if you feel the gain
is too high. I suspect it is only because today's modern devices have
far higher outputs, thus your struggle with the volume knob. I suggest
that if this is the case, and you believe the gin is too high, then go into the
source devices feeding this preamp and check the back for output gain
and lower if there is such a control. it would be a crime to mess with
the preamp. although when the spilt window corvette coupe of 1962 (only)
was introduced, there were folks that removed the split window thereby
destroying the value of the 1963 corvette coupe.

another way to lower gain is to use the tape monitor to pass all signals
(meaning the tape monitor is always engaged) to an external box with
super high quality film deposition resistors (per channel) to reduce
the signal. this assumes your preamp has a tape monitor circuit.

if the new tubes do not reduce the hiss, then the next step is to
replace the resistors. first off, ALL must be measured to ensure
they are within margins of the original design. you will need to
find period-correct AND identical to the original vendor in specific
product line, value, wattage, voltage handling, size, etc.
if you don't use original resistors, and for example, you
use modern metal films (at 1W or better) then the sound
will change. as well as resale value.

there are more changes but we'll leave them out for now.

the key is how much effort needs to overcome the hiss
annoyance.
Click to expand...

Thanks for the reply. I am unable to move my equipment around due to shoulder surgery 2 weeks ago. prior to surgery I tried the pre with 2 different amps and had same noise with both. As mentioned above it makes same noise regardless of source (I had a mcintosh d150 dac and tt hooked up). The dac has 2 sources streamer and cdt going thru it. I tried the phono thru the vtl's phono stage and also thru my mcintosh mp100 phono stage. All the different setups produced the same noise. I need to clarify that it has 2 12ax7's and 4 12at7's but the phono stage has 4 12ax7's and 2 12at7's for a total of 12. I bought this used and hodge podge of tubes. on the non phono side it has ge 12ax7's and in place of the 12at7's it has cv4024 KODD/K. On phono side it has 2 cv4004 KODD/K for the 12ax7's, 2 I can't read where 12ax7's are suppose to be and 2 cv4024 KODD/K for the 12at7's.
Regardless if the tubes are the problem I want to replace them. I am just looking for best brands anf options to the 12ax7's and 12at7's. So to summarize I need 6 12ax7's or equivalents and same on 12at7's.
 
couple of questions, Does anyone have thoughts on replacing 12ax7's with 5751's and what brands to look for? If replacing with 5751's do you still use 12at7's in the other spots?
 
let's start over again. do the tests (3) I mentioned earlier. since you did not mention the
other tubes in the phono section, test the phono section by outputting via tape
monitor (is there one or more tape monitor circuits?). if NO noise then
eliminate the phono section as the source of the noise. if you do not listen to LPs
then skip all references to the phono section.

it does not matter what the brand of the tubes are. for example, a 60YO GE tube that's
been in service in a circuit that pushes the limits might be worn out. and a cheap chinese
12ax7 might outperform it. and unless you are willing to spend incredible amounts
of money on NOS Telefunken or Mullard on 6 12ax7s, then skip the bing/brands
and concentrate on finding which tubes are causing the noise issue. skip the gain thing.

if you eliminate the phono section as the cause of the noise, then you have FOUR
additional tubes to play with: you can be assured that if the phono section is not
the source of the noise, then you can assume the tubes do not. then replace
both 12ax7s in the line stage, and if the noise persists, then it's not the 12ax7s and
you are now down to the 4 12AT7s. that's an elimination of 8 tubes as the source
of the noise.

your best bet is to buy a full set of tested tubes and simply replace them in your preamp
if you don't have time/effort to trace the noise down. this is the super fastest way of
isolating the noise.

depending on how much money you have, it's either expensive NOS like Mullard or
telefunken, then the older RCA/GEs, and if money is an issue then you can
buy Russian equivalents.

the use of 5751 is IFF you have exhausted all the ideas I gave you earlier and
have gone through the schematic and design and have come to the conclusion
that the ONLY solution to the gain issue is to use 5751s. Otherwise it is a waste
of time, and money - money that is better spent on that full set of known good,
tested tubes. and if you believe that the only solution is the 5751s, the Russian
ones are good.

BTW if you are not willing/able to troubleshoot then a full set of tubes is the
only way forward.

and you need to decide which is more important, this gain issue or the noise
issue. money spent on the former means less on the latter.
 
Bob said:
let's start over again. do the tests (3) I mentioned earlier. since you did not mention the
other tubes in the phono section, test the phono section by outputting via tape
monitor (is there one or more tape monitor circuits?). if NO noise then
eliminate the phono section as the source of the noise. if you do not listen to LPs
then skip all references to the phono section.

it does not matter what the brand of the tubes are. for example, a 60YO GE tube that's
been in service in a circuit that pushes the limits might be worn out. and a cheap chinese
12ax7 might outperform it. and unless you are willing to spend incredible amounts
of money on NOS Telefunken or Mullard on 6 12ax7s, then skip the bing/brands
and concentrate on finding which tubes are causing the noise issue. skip the gain thing.

if you eliminate the phono section as the cause of the noise, then you have FOUR
additional tubes to play with: you can be assured that if the phono section is not
the source of the noise, then you can assume the tubes do not. then replace
both 12ax7s in the line stage, and if the noise persists, then it's not the 12ax7s and
you are now down to the 4 12AT7s. that's an elimination of 8 tubes as the source
of the noise.

your best bet is to buy a full set of tested tubes and simply replace them in your preamp
if you don't have time/effort to trace the noise down. this is the super fastest way of
isolating the noise.

depending on how much money you have, it's either expensive NOS like Mullard or
telefunken, then the older RCA/GEs, and if money is an issue then you can
buy Russian equivalents.

the use of 5751 is IFF you have exhausted all the ideas I gave you earlier and
have gone through the schematic and design and have come to the conclusion
that the ONLY solution to the gain issue is to use 5751s. Otherwise it is a waste
of time, and money - money that is better spent on that full set of known good,
tested tubes. and if you believe that the only solution is the 5751s, the Russian
ones are good.

BTW if you are not willing/able to troubleshoot then a full set of tubes is the
only way forward.

and you need to decide which is more important, this gain issue or the noise
issue. money spent on the former means less on the latter.
Click to expand...

Question; can the phono board tubes cause noise when not playing phono? I wouldn't think it could. Is it safe to play pre without any tubes in phono section? What are these -cv4024 KODD/K and 4004? I . Please don't take it that I don't appreciate you efforts and advice because I do, its just I am unable to move equipment around with 1 arm for the next month or so.
 
one arm is sufficient to pull, move, replace tubes, even if it's one-at-a-time.
in fact, with tube amps and their high and lethal voltages even when
powered off, it is a one handed affair and the non-use of the left arm
is recommended.

CVxxxx is a European designation. you can look them up and decide.

not sure all the information needed to generate advice is all there. the best bet
is to post the schematic and the exact physical tube, in terms of labelling,
in each position.

without the knowledge of the (only) 3 tests and results, and the
subsequent ability to move tubes around to isolate the section
and the tube type, then it's guesswork all around.
 
Bob said:
one arm is sufficient to pull, move, replace tubes, even if it's one-at-a-time.
in fact, with tube amps and their high and lethal voltages even when
powered off, it is a one handed affair and the non-use of the left arm
is recommended.

CVxxxx is a European designation. you can look them up and decide.

not sure all the information needed to generate advice is all there. the best bet
is to post the schematic and the exact physical tube, in terms of labelling,
in each position.

without the knowledge of the (only) 3 tests and results, and the
subsequent ability to move tubes around to isolate the section
and the tube type, then it's guesswork all around.
Click to expand...
The one arm is about moving equipment not tubes
 
Last edited:
any reason you need to move equipment to test a set of tubes?

I assume, although it might be a bad assumption, that everything is all set up
and all that's needed is to pull/move a few tubes.
 
Bob said:
any reason you need to move equipment to test a set of tubes?

I assume, although it might be a bad assumption, that everything is all set up
and all that's needed is to pull/move a few tubes.
Click to expand...
actually the pre is not in use so I will have to move it to hook it up. It is light enough though so I can move it with one arm. I will try moving some around some tubes this afternoon and see if it makes any difference. Will a bad tube in phono section affect pre when not using phono?
 
not generally but more information is needed.
however, if you suspect a bad tube, then move it from one channel to the other:
if its bad the problem moves with the tube, if not chances are, both are good.

but, you'd need to follow the schematic to see if there are complementary
tubes in the two channels. eg 5th paragraph in post 2. so the answer does depends
 
you can cut and paste from webpage, or use browser's tools, or use the windows tools,
or the automatic tools from oneNote.

I am a firm believer that if you catch a fish and feed a man, he starves tomorrow,
but if you teach him to fish, he feeds himself forever.

try this, and if this doesn't work for you, search "cut and paste":

http://tips4pc.com/basics/how_to_copy_and_paste.htm

there is so much more work ahead if we skip the tube swapping
and go straight to the schematic. doing both is OK
 
Last edited:
last night I swapped 2 of the 12ax7's from the phono board to the line stage and it was noticeably quieter after playing for 30 minutes or so. I had not played it that long since I got it so I am not sure if it was letting it warm up that much or the tube switch. I did not try phono stage yet due to location of the TT and pre. I will try that next.
 
good - now we're in gear.

fix one (phono or line stage) at a time, else you risk continuous movement of
one or more bad tubes (if tubes are the culprit) but you can eventually reduce
the noise contribution from the tubes.

please be anal, write down everything but with a plan. label on paper
each and every original tube, then one at a time, move then and
note the noise.

you won't get anywhere randomly. since there are 6 of each type of tubes,
you have about six factorial combinations for each. that's over 1000
possible tube locations.

you can do this simply.
 
I wasn't able to hook TT up but when turning pre to phono it got more than noticeably louder. When I have more time I will switch tubes back 1 at a time and try to narrow it down to 1 tube.
 
do it the other way. one at a time, repeat for each tube type, put into test channel
and mark for that tube the noise level, low, medium, high. then at the end
test each level and order 1,2,3,4,5,6 by noise level.

do this for both sets, then move the lowest noise tubes into the line stage
and run music. if it's lower then you have a test scenario where tubes
contribute to the noise. next step is to purchase one or more new
tubes ( and select tube types known to be lo noise) then substitute
the old, even low noise, for the new ones.

this is the lowest possible noise using the lowest noise tubes.

any noise level lower than this will require getting inside the device,
following the schematic and identifying the areas where you may need to
inject a signal and compare before and after to determine whether
that stage is noisy or not.
 
if i can get the noise to the point of having to put my ear to speaker to hear I am good. It is close now. I have been looking at NEW tubes, whats you thought on new vs NOS and how long is burn in on New tubes?
 
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