Herbert gain and output fading out intermidently

[RJF]

My Herbert, for the first time in 7 years of ownership, appears to have a problem.

All of a sudden the amount of gain went down to about half of normal on ch2 and 3, and channel one had very little output and sounded scratchy. If I wait a few minutes, everything goes back to normal. It did this cycle twice on me during a playing session.

I would assume preamp tube? Anyone have an idea which slot it would be?

 

[Peter Diezel]

If your tubes are 7 years old it´s time to replace the whole set.

 

[RJF]

I did a complete pre/power tube change about 5 years ago. The thing is, I've probably only put less than 100 hrs on it in the last 7 years. I don't play very much. But yeah, it's time to go with some new tubes. I plan to go with a pair of KT88's and two pairs of KT77's.

 

[Peter Diezel]

New tubes can be bad.

Did you re bend the clamps of your tube sockets ?

 

[Black Sabbath]

I did a complete pre/power tube change about 5 years ago.

Please listen to what the man says.

If your tubes are 7 years old it´s time to replace the whole set.

New tubes can be bad.

I plan to go with a pair of KT88's and two pairs of KT77's.

Why mix power tube types together?

Most likely at that point you're simply splitting hairs and nobody will notice a difference in tone but possibly you. To me it goes against the whole idea of getting good uniform tone. In my opinion the only reason Peter has made this feature available to use in his amplifiers is for convenience.

I will dare to say that Peter himself may not like to have a 16, 25, 34 (Herbert) mix of power tubes in his amps. To a certain extent a non-harmonized configuration. It then can become rather monotonous.

Anyway, it's well known that Herbert doesn't value confusion.

 

[diezel&gas]

If your tubes are 7 years old it´s time to replace the whole set.

Hi Peter, I'd like to change power tubes in my Herbert, can I buy the matched sextet set from you?
What does the kit include? Power tubes only or power and preamp tubes? Danke!

 

[RJF]

I did a complete pre/power tube change about 5 years ago.

Please listen to what the man says.

If your tubes are 7 years old it´s time to replace the whole set.

New tubes can be bad.

I plan to go with a pair of KT88's and two pairs of KT77's.

Why mix power tube types together?

Most likely at that point you're simply splitting hairs and nobody will notice a difference in tone but possibly you. To me it goes against the whole idea of getting good uniform tone. In my opinion the only reason Peter has made this feature available to use in his amplifiers is for convenience.

I will dare to say that Peter himself may not like to have a 16, 25, 34 (Herbert) mix of power tubes in his amps. To a certain extent a non-harmonized configuration. It then can become rather monotonous.

Anyway, it's well known that Herbert doesn't value confusion.

Well, I thought that was one of the great things about a Diezel is that you can run multiple pairs of tubes. I guess I will try it out with all KT77's then. In the past I have really liked the way it sounds with '88's, especially on the bottom end, but feel the amp is starting to miss something. I don't think it sounds like it used to, plus it's biased on the cold side at 50ma as per Peter's advice as the big bottle tubes are harder on the amp I guess.

One thing I notice is it doesn't sound "in your face" anymore. Meaning, it sounds like when you plug a 16ohm cab into the 8 ohm port of an amp. I'm hoping fresh tubes and a hotter bias brings it "back to life."


I have two questions for you Peter.

Somehow the bias got out of adjustment a few years ago and went real hot on one pair of tubes. I noticed that the volume wouldn't get any louder past about 10 o clock on the master volume and I could smell a burnt smell after playing a little while. Smelled like it was coming from the small "transformer" looking item that sits on top the chassis, right behind power/run switches. I immediately found the hot pair and got the bias adjusted back to 50ma ( it was somewhere around 150ma IIRC). I have checked it a few times since and it has not changed again. Amp seems fine overall, but thought I'd check what you thought about it.

2. Does someone have the schematic or list that tells which preamp socket is which?

 

[RJF]

Finally found the preamp tube list through searching.

Looks like my culprit is likely in V1, or V3.

V1 Used by all channels : Ruby 12AX7AC5 HG+
V2 Used by channel 3 : Ruby 12AX7AC5 HG
V3 Used by all channels : Ruby 12AX7AC7
V4 Mixer & Loop Driver : Ruby AC5 select
V Return Driver : Ruby AC5 select
V6 Phase Inverter : Sovtek LPS

 

[Peter Diezel]

Gentlemen,

since I´m not on the forum every day send me an email
to peterdiezel (at) diezelamplification (dot) com

Thanks,
Peter

 

[diezel&gas]

ok, i'll drop you few lines!

 

[Black Sabbath]

I don't think it sounds like it used to, plus it's biased on the cold side at 50ma as per Peter's advice as the big bottle tubes are harder on the amp I guess.

I presume you mean 50mA per pair! Doooodd . . . that's a wee way too cold for Herbie. Even 60mA per pair is too cool but that's what Peter recommends by way of the sticker on the back top end of the chassis on Herbert - see attachement.

I think that Peter out of his good heart is simply making sure you don't blow out power tubes unnecessarily (too quickly).

The biasing of Herbert goes something like this:

You either can use the 0.6 (60%) factor or use the 0.7 (70%) factor. 0.7 will cause a better clean sound while 0.6 will cause a better tight and percussive sound.

The EL34 has 25 watt of maximum plate dissipation:

25W x 0.7 = 17.5W

17.5W / 480V (plate voltage) = 0.036A = 36mA each tube. To be safe against low main voltage and to stay out of cross over distortion set a pair of EL34 power tubes to 70mA.

 

[diezel&gas]

my actual tubes EL34 ( 2009 circa ) were like to 52 mA per pair, on the cold side too.
does bias value go down after few years of wear? i raised it a bit to 62.7/8 mA per pair now. i didn't go further because i thought tubes might fail very quickly since they are "old"

 

[RJF]

I don't think it sounds like it used to, plus it's biased on the cold side at 50ma as per Peter's advice as the big bottle tubes are harder on the amp I guess.

I presume you mean 50mA per pair! Doooodd . . . that's a wee way too cold for Herbie. Even 60mA per pair is too cool but that's what Peter recommends by way of the sticker on the back top end of the chassis on Herbert - see attachement.

I think that Peter out of his good heart is simply making sure you don't blow out power tubes unnecessarily (too quickly).

The biasing of Herbert goes something like this:

You either can use the 0.6 (60%) factor or use the 0.7 (70%) factor. 0.7 will cause a better clean sound while 0.6 will cause a better tight and percussive sound.

The EL34 has 25 watt of maximum plate dissipation:

25W x 0.7 = 17.5W

17.5W / 480V (plate voltage) = 0.036A = 36mA each tube. To be safe against low main voltage and to stay out of cross over distortion set a pair of EL34 power tubes to 70mA.

I remember a few years back, Peter didn't recommend using 88's or 6550's anymore, for whatever reason. But if we were, to set the ma at 50 per pair.

I am going to go 70ma per pair, and I am going to try 2 pairs of KT77's, and one pair of 88's. THen I will try all KT77's. I'll go with what sounds best between those 2.

 

[diezel&gas]

Kt88 are loud and need to be cranked a lot to sound good I read

 

[RJF]

Kt88 are loud and need to be cranked a lot to sound good I read

Yes they are loud.

No they don't need to be loud to sound good. That has to do more with the amp itself than the tube type IMHO.

VHT UL, Bogner Twin Jet, my Mesa rev F with 88's, all sound good at low volumes.

 

[Black Sabbath]

my actual tubes EL34 ( 2009 circa ) were like to 52 mA per pair, on the cold side too.
does bias value go down after few years of wear? i raised it a bit to 62.7/8 mA per pair now. i didn't go further because i thought tubes might fail very quickly since they are "old"

My rule of thumb, fundamental heuristic, trial and error, which ever you prefer is to change EL34 guitar amplifier power tubes every 1000 hours or every year - whichever comes first. Your mileage may vary.

I remember a few years back, Peter didn't recommend using 88's or 6550's anymore, for whatever reason. But if we were, to set the ma at 50 per pair.

I am going to go 70ma per pair, and I am going to try 2 pairs of KT77's, and one pair of 88's. THen I will try all KT77's. I'll go with what sounds best between those 2.

A quick and general overview of the most common (single) tube types for use in the
Herbert:

EL34/6CA7: Aggressive, slender low end, higher total harmonic distortion.
Recommended bias setting: 30-35mA

5881/6L6: Great tube for clean sounds, good bluesy tones, also has strong bass and excellent clarity.
Recommended bias setting: 25-35mA

6550: Very loud, very good dynamics, strong low end, amazingly percussive.
Recommended bias setting: 40-60mA

KT88/KT100: Similar to 6550, but very punchy with great clarity and warmth - expensive!
Recommended bias setting: 35-50mA

 

[diezel&gas]

Kt88 are loud and need to be cranked a lot to sound good I read

Yes they are loud.

No they don't need to be loud to sound good. That has to do more with the amp itself than the tube type IMHO.

VHT UL, Bogner Twin Jet, my Mesa rev F with 88's, all sound good at low volumes.

Good to know I read it, so we bust a myth
I read they need very power amp push to distort and they're really loud

 

[diezel&gas]

I wish I had them all to try not possible now

 

[RJF]

Kt88 are loud and need to be cranked a lot to sound good I read

Yes they are loud.

No they don't need to be loud to sound good. That has to do more with the amp itself than the tube type IMHO.

VHT UL, Bogner Twin Jet, my Mesa rev F with 88's, all sound good at low volumes.

Good to know I read it, so we bust a myth
I read they need very power amp push to distort and they're really loud

They are indeed really loud. My dual rectifier with them is louder than a stock triple rec.

The whole point of using 88's in a modern metal amp is to keep the power amp from distorting. Overdrive from the power section is not needed or wanted.

 

[RJF]

my actual tubes EL34 ( 2009 circa ) were like to 52 mA per pair, on the cold side too.
does bias value go down after few years of wear? i raised it a bit to 62.7/8 mA per pair now. i didn't go further because i thought tubes might fail very quickly since they are "old"

My rule of thumb, fundamental heuristic, trial and error, which ever you prefer is to change EL34 guitar amplifier power tubes every 1000 hours or every year - whichever comes first. Your mileage may vary.

I remember a few years back, Peter didn't recommend using 88's or 6550's anymore, for whatever reason. But if we were, to set the ma at 50 per pair.

I am going to go 70ma per pair, and I am going to try 2 pairs of KT77's, and one pair of 88's. THen I will try all KT77's. I'll go with what sounds best between those 2.

A quick and general overview of the most common (single) tube types for use in the
Herbert:

EL34/6CA7: Aggressive, slender low end, higher total harmonic distortion.
Recommended bias setting: 30-35mA

5881/6L6: Great tube for clean sounds, good bluesy tones, also has strong bass and excellent clarity.
Recommended bias setting: 25-35mA

6550: Very loud, very good dynamics, strong low end, amazingly percussive.
Recommended bias setting: 40-60mA

KT88/KT100: Similar to 6550, but very punchy with great clarity and warmth - expensive!
Recommended bias setting: 35-50mA

That makes me want to stay with KT88's. What about KT77's?