My amp powers up and makes some pretty bad noises. The Gain and Master Volume appear to be working pretty well, but I am not getting anywhere near full volume. I can plug in my guitar and hear my playing through the speaker, but I have to crank up the Master Volume to at least 1/2 way and the Gain all the way up to make any sort of real sound. But, if I strum a loud chord or anything loud, it makes a very loud crackling/popping noise. Also, if I turn the Middle or Treble controls, I can actually dial in a high pitched hum that is completely uncomplimentary to my playing. This is very frustrating. I have checked my connections and my tubes appear to be good. I biased the EL34s as is and am running them at about 17.5 W. It just sounds HORRIBLE. I am pretty frustrated so do you guys have any suggestions? I dont know what bad tubes sound like, so I am kinda lost...
Thanks in advance!
Also I attached the updated schematics.
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I think this may be the problem. Take a look at C14. This is the decoupling cap for the grounded grid side of your phase inverter. You've got it connected between the grid and the bottom of the bias resistor R18, on top of the 10K tail resistor R21. Try connecting it directly between the grid and ground. If you had negative feedback, it could sit on top of the feedback shunt resistor.
This next thought won't make much difference but here goes. You've got the suppressor grids connected to the top of the 10R cathode resistors so they will have a small positive voltage. You want them as negative as possible so I'd move the connection to ground (or you could be clever and connect them to the bias supply before the 15K resistor and make them really negative.)
"T" on the preamp section should connect to ground below R21 10K tail resistor. Actually I think C14 should be dawn on the preamp section as it belongs with the PI. With "T" wired as you have it drawn your master vloume pots are 10k abouve ground and floating on the PI bias network. We dont have a picture to confirm that it is wired the way its drawn.
I have changed my schematic accordingly! Thanks guys! I am afraid I made an error in the schematic that I unfortunately followed in my wiring. Ill get to that change in the morning! More on that later!
On a different note...
Firestorm wrote:
This next thought won't make much difference but here goes. You've got the suppressor grids connected to the top of the 10R cathode resistors so they will have a small positive voltage. You want them as negative as possible so I'd move the connection to ground (or you could be clever and connect them to the bias supply before the 15K resistor and make them really negative.)
I understand that during the bias process, you want to achieve the highest negative voltage, but once I have biased the tubes to the appropriate wattage, what does more negative change?
I was talking about g3 of the EL34s (pin 1). The supressor grid reflects secondary emission electrons back toward the plate. The more negative it is, the better it works. Per KO'C, Peter Traynor connected it to the raw bias supply instead of ground and improved tube reliability.
Like I said, it won't make a huge difference, but I'd connect pin 1 to ground instead of the top of the 10R Rk.
I think you guys are right, I got the 10R idea from a schematic I saw that ran EL34s and used the 10ohm resistors on top of the bias. I liked the sound clips, so I just wanted to check it out! haha. I am not attached to them, so I think I'll just remove them. I kind of like having some kind of Cathode Resistor there because it makes biasing a lot easier!
Well yeah, the 1 ohm resistors are purely there for measuring bias.
If you use another technique to set the bias you don't need them there at all.
But it is a safe and simple way for most.
It would be safer yet to have them wired to test point jacks on the back panel.
If you are going to do that then you may as well use a bias pot that can be adjusted from the exterior as well.
Structo wrote:Also, Firestorm not sure I follow what you are trying to describe?
Don't most EL34 amps have pins 1 & 8 connected together?
Yeah, most. Biut since EL34s are one of the few tubes that give you a separate pin for g3 (instead of connecting it to cathode internally) you can do some things differently. The suppressor grid was added tetrodes (making them pentodes) because electrons knocked loose from the plate can be attracted to the screen grid instead of back to the plate, especially when the plate voltage swings negative WRT to the screen (as can happen more easily when a tube is biased hot). With g3 connected to ground, the zero potential space charge around the suppressor grid steers most of the secondary emission electrons back to the plate, increasing plate current and widening the useable plate voltage swing.
This works fine, but what Traynor did works better by making g3 even more negative by connecting it to the raw bias supply. That makes the tube more efficient AND helps prevent catastrophic damage in the event of bias failure by having a second negative element in the tube to prevent runaway.
I don't think skyy has to go that far, but with the 10Rs on the cathodes, g3 is at a slight positive potential. They work better at zero or below.
You can run your 1R (or 10R) between pins 1 & 8 right on the socket, then ground pin 1. That way, your G3 is still at ground potential and your cathode is raised to check bias.
Guys! You rock! I got the amp making actually pleasing noises. Overall volume seems a little low still, but I am going to try rebiasing the amp now. Hopefully that will get it sounding great!
As of right now, I am down to 3 problems left before this sucker can sing!
1. In order to come anywhere close to the Volume of a 40ish W amp, I have to turn the GAIN and MASTER VOLUME all the way up.
2. In certain positions with the MIDDLE, TREBLE, and GAIN pots, I can dial in a lovely crackling noise. It's great!
3. My EQ section is officially for show. The BASS is acting like a volume... If the TREBLE or MIDDLE are turned all the way down, no signal passes through. The TREBLE and MIDDLE pots both have "sweet spots" where both they have dramatic swells while spinning the pots.
The BASS is acting like a volume... If the TREBLE or MIDDLE are turned all the way down, no signal passes through. The TREBLE and MIDDLE pots both have "sweet spots" where both they have dramatic swells while spinning the pots.