not matched set of tubes (noob question)

[mat]

I havent found a answer for the following question that I started to think yesterday:
What would be the symptoms for replacing one bad EL84 in a matched quad (cathode bias) for a different new EL84 ? Can it harm the amp or is it just sound problem ?
I just bought such amp and one EL84 is finished. I replaced the set with new JJ's and the sound is not nearly as good as with the old tubes (despite the ocassional howling/noise problem).

[talbany]

Mat
As long as they are not red plating on you it shouldn't harm the amp.. Mis- matched tubes of any kind in a push pull amp will introduce (i believe) more 2nd order harmonics so the amp might not be as smooth and sustain and bloom as a new set of matched quad,so yes it can effect tone..The further out of balance they are more you will notice it..On the flip side you might like the way it breaks up..Just keep an eye on it to make sure they don't run away on you and damage the output transformer..I would check to see how hot those are running,you can use the weber bias calculator..EL-84's generally sound best run around 90-95% but that sometimes leads to slightly shorter tube life..
BTW..Your new quad didn't bias up the same as your old ones so that is why they don't sound the same!..You could install the new quad and run the numbers and adjust the cathode resistor to match the old set and chances are get them to sound closer to the old set..If you want to go through the trouble..

http://www.webervst.com/tubes1/calcbias.htm
Good Luck!

Tony

[Stevem]

The queation sound wise is somewhat related to if the amp is biased class A or not.
If its class A/B and the match is better between the new quad to tubes than there will way less even order harmoincs getting out to the speaker and your ears.

[martin manning]

In my experience JJ's are pretty good sounding EL84's. Can you measure the current on each side of the OT to see how unbalanced you are with the one new tube installed? If you have some differences in the individual tubes you may be able to rearrange them and get a better balance. If the match is really bad, you will hear an increase in hum.

[pdf64]

Tubes don't tend to red plate until significantly above their limiting dissipation, may be 150-200% depending on the type.

So though red plating should set the alarm bells ringing, the absence of red plating does not indicate suitable dissipation levels.

With shared cathode resistor biasing, the bias voltage depends on all tubes conducting at a nominal level.
If one tube is dead (ie not conducting) the absence of the contribution of its current will lead to the cathode (ie bias) voltage being lower and the remaining functional tubes will therefore conduct more heavily, thereby leading to increased plate dissipation.

I don't see how the class of operation will significantly affect the OP query?
Amps using p-p EL84 in cathode bias tend to run them very hot and the class of operation becomes a technicality.
Pete

[mat]

Thanks Tony and all for the answers. I will have time to further study the amp on my week off after one week. I have one extra old EL84 that I can try. Will do the math also.

The amp is Crate Vintage Club 30 which is a PCB amp with all tube sockets, pots and jacks soldered to the PCB. Did replace few pots and two jacks already. I also will do the cab job although the electrolytes seem to be OK. Will also have to make a box for it being just the chassis I bought. The PT seems to be little rusty but tube sockets and other parts are in good condition.

The reverb is quite subtle but it might be the original design. I dont have the original pan so it might be that also.

Links to the schems:
http://www.blueguitar.org/new/schem/mis ... 112pre.gif
http://www.blueguitar.org/new/schem/mis ... 112pow.gif

I must say the amp is really joy to play as seem to be the case with quite a few other players also. The reliability is another story..

Here is what the designer of the amp has to say about the Vintage Club series:

Hey guys,

No doubt the Original VC amps (blonde and black) had their limitations... but understand the price point Crate was trying to hit on the thing. It was to be $599 retail in 1996. A lot of compromises needed to be made. The jacks at the time would oxidize from non use and cause switch contact issues, pc boards were cheap single sided affairs, many parts were sourced from Asia, I was put on the project to get the Belton reverb pans to sound good in our circuits, worked with Dr Park extensively on his pans, trying to get high end respnse in them and get the decay time down to a medium level, the IC drive and recovery circuit...well, its a cost thing...plus that method of incorporating reverb into an amp is unique in that it does not alter the basic signal path architecture, so a compromise for the reverb to improve the basic sound. additional stages in the signal path would have changed it entirely. The wave solder 9 pin socket was another joint development with Belton and look at how many are being used in Mesa's and other hi-end amps today....you guys are lucky to have the Chicago made OT in there....94-268-01, that is a great sounding transformer...Don Sokolis at Electrical Windings cloned a tweed tremolux tranny for me....The power ratings, well, that's the marketing guys for ya, but all in all, after all these years, you can still find them for dirt cheap and the issues can be easily solved. The only difference between the cream and black ones was a MF resistor in the screens instead of a wirewound. Maybe some component sourcing changes as purchasing would always be scouring the earth for a cheaper part...sometimes R&D would get a chance to comment, other times it just happened.

But when you plug into that clean channel of an old VC3112 and it sings,bounces and glasses like an old Deluxe reverb...hard to justify NOT buying it for $250. The dirty channel was subject to many mods, even back then. One guy wants AC/DC, another wants Robben Ford...fiddle until it sounds right.

That amp came about because "they" asked me why I didn't play a Crate, and I told them you don't make one that sounds right...so the challenge was put to me to design a Crate that I would play through....and I did.

Lots of good times and experience with that group of guys, too bad it all got flushed down the toilet by Loud.

I had nothing to do w/ the V18, etc....other than be forced to give up the schematics and work with the Chinese engineers that didn't even play guitar or understand the music and sound. Sorry if they turned out sounding like crap, hey, at least they are China cheap.
__________________
Obeid Khan-Design Engineer Reason Amps www.reasonamps.com

Amps I have designed - Crate Vintage Club, Vseries, Palomino, Ampeg Jet, Reverbrocket, SuperJet, B15R, Ampeg J20 (ptp) SVTDI, SVTMP

[talbany]

Mat
I've had a few cross the bench and too thought they were pretty good sounding amps for what they were

In general cathode biased EL-84 amps (biased Hot AB1) generally change in tone after a tube replacement and given the fact that they might all draw current differently (therefore bias differently)..When we read about various tube reviews on EL84's using the same amp and same value cathode resistor some might bias hotter or colder than the last set so the sound changes (similar to that of a pre amp tube) so IMO these kinds of reviews can be a bit misleading (unless you change cathode resistors to run at the same idle dissipation)..It's always a good idea to run the numbers after a change to give you a reference point as to how you like the tone as well as how hot or cold they might be biased..Of coarse different tubes will sometimes sound a bit different but the goal is to get as close as possible to try and shoot for the same tone you were getting before you swapped them..
BTW.. Your amp is NOT running in true class A operation!!

Good Luck

Tony

[Stevem]

Those output tube sockets on those amps and the solder that holds them to the board always goes bad with expansion and contaction of heating and cooling sessions.
While you are in there be sure to suck all the factory solder out from the connections and reflow in leaded type solder that is far lees likly to form bad connections on ya.

[mat]

I found time to measure the voltages from the amp and they seem to be very near each other with old and new set of EL84's.

Rc=61.2ohm

JJ's aV= 313V, 313V, 315V, 315V
old set= 315V, 315V, 316V, 316V

JJ's between anode-cathode 301V, 301V, 303V, 303V
old set between anode-cath. 307V, 307V, 307V, 307V

Voltage across cathode resistor:
JJ's 10.4V
old set 10.2V

Weber calculator gives 40mA for the plate current.

I did again a tap test for the old set and it seems only one of the tubes is working properly. Very gentle tap to other tubes made the amp howl.

[mat]

Those output tube sockets on those amps and the solder that holds them to the board always goes bad with expansion and contaction of heating and cooling sessions.
While you are in there be sure to suck all the factory solder out from the connections and reflow in leaded type solder that is far lees likly to form bad connections on ya.

I did allready reflow and added extra solder to the tube pins. The solder connections seemed to be in a very good condition. Maybe this amp dont have too many playing hours behind..

[rp]

Tubes don't tend to red plate until significantly above their limiting dissipation, may be 150-200% depending on the type. So though red plating should set the alarm bells ringing, the absence of red plating does not indicate suitable dissipation levels.

That's a seriously good tidbit to have stumbled upon. Thanks.

[martin manning]

Pete are you saying that running continuous plate dissipation of 150-200% in most cases wont result in redplating?

[mat]

So is 40mA normal on this kind (AB1) of design ?

[Phil_S]

So is 40mA normal on this kind (AB1) of design ?

In a cathode biased amp, tubes will settle in at idle at a self-selected point that is influenced by the choice of cathode resistor for which 60 ohms would be considered a standard value. Certainly 40mA per tube seems reasonable.

[mat]

So is 40mA normal on this kind (AB1) of design ?

In a cathode biased amp, tubes will settle in at idle at a self-selected point that is influenced by the choice of cathode resistor for which 60 ohms would be considered a standard value. Certainly 40mA per tube seems reasonable.

Thanks for the quick answer. Might try the amp with Rc 50R

The box is almost ready. Simple plywood with a mahogany from a old boat for the front wood. I'll post pics when ready. The whole project has strong recycling wibe on it. Plywood, mahogany, chassis -all salvaged from going to trashes.
One thing missing is the handle. Any suggestions for it welcome !