freestompboxes.org

Phatt, how does the TC rating change the tone? I understand theyre really different but I'm just not certain what that changes

Hi again,
El84 only needs a 30VAC wiggle on the grid to produce full power whereas a 6V6 needs closer to 70VAC. Some of the really big power tubes need even more. This affects how the power comes up,, fast or slower.

Build two identical PP Amps, one with 6V6 and the other with EL84.
The EL84 Amp will distort long befroe the 6V6,,,Depends greatly on what you want from an amp.
If You want super clean then obviously the 6V6 will appeal to ya.

There is a clue to this, just look at marshall then fender pp schemo's an notice how quite often the PI in fenders use an *AT7* for the PI that's because the tetrodes tend to need a little more grunt to get them going. obviously the Real Pentodes
don't need much help.

As to tone ??? well daz a loaded Q depends who you talk to?
IMO (having done the above mentioned experiment) EL84 has more bottom end than 6V6.

But don't let that fool you it depends how well you understand the technoligy. You can get great bass from a 6V6 if you know how to tweak the tubes in use. Same with distortion they can sound fat and muddy just depends how you setup the circuit.
This is why most Valve tests are a no brainer. Just because a particular valve does not work well in one circuit does not mean it's a crap valve as it may work brilliant in another circuit. You really have to work a lot with them before you catch on to these hidden gems.
Phil.

Hey phatt, got any suggestions on how I should bias it? Are there any mods I can do now to make biasing easier or more accurate?

On just about every schematic from those early fender circuits is a little note at the top which says;
"ALL voltages + - 20%"
Yet lots of folks get all funny bout exact bais and perfect balancing. Some of the greatest amps likely had bias voltages that where not perfect so go figure?
You would only need to consider strict bias and balance if you where building a monster Bass rig (below 100Hz) THEN it makes sense to have matched tubes and all the fancy stuff. For the purpose of guitar signals it only makes the owner feel
better if he has perfect balance and bias.
Just bias to the specs of the Valves you use. Running hotter bias for better tone is another one of those old guitar player fantasies it does very little to run them above the book values.

Some imbalance may well create a more pleasing distortion of the signal and this can be useful.
In Fact some harp amps unbalance the PI (or something along those lines) to extract more distortion from the Amp.

The trim pot bias verses the fixed resistor is a 50/50 bet.
trim pots can over time go open circuit and this can destroy the power section, at least fixed R values will stay put.

If you are building from scratch then pop in a couple of 10 Ohm resistors from Cathodes to ground. (that will save you the need to buy a bias probe) When time comes your bias probe is built into the circuit so just measure the CS test points then alter the fixed Resistor values to allow for any changes in bias.

Please note; Those 10R CS (current sense) resistors should be **1% tolerance** just use half watt metal film.

Most will say use 1 Ohm for CS but the average DMM may struggle with such low value, you will get better accuracy by using 10 Ohm. Just remember to move your decimal point across one digit when you do the maths.

Also I've seen some people using 5 watt wire wound R's as CS resistors,, unlikely you will get an accurate reading from wire wounds. LOL
They only need be 1/2 watt 1%.
Phil.

Well, I got it all wired up today and turned it off standby and it made the most aweful noise I thought was gonna blow my speakers. REALLY loud sound almost like oscilation. Any idea what could be causing that? I was hoping it would be something simple and stupid but I'm afraid it wont be simple or a quick easy fix.

Sounds like you have your OT secondary leads backwards. You can reverse OT leads either on the primary or the secondary and it should work.

I usually power up a newly built amp with a variac the first time so if it goes into oscillation I don't end up needing to change my underwear...

TT

Yeah ha I feel like I need to change mine right now. Scared the daylights out of me. So, when I turn standby off, the Power Tubes started doing a weird blue glow thing and it appears that is what's causing the sound. Is that because of the leads?

And would it make a difference if I took the EXT speaker jack and ran the feedback resistor and the secondary lead to the tip of the output jack?

Okay, I got it to stop oscilating. I guess my bias was so hot that paired up with the oscillation, it was just driving my tubes like crazy. Now, they're still blue and they're redplating. I've reduced the value of the bias range resistor from 470 to 100. It's still red-plating. should I keep reducing the resistor? It only hovers from around 55 to 63 or so. hm

Well it's working kinda. Still got a low hum to it and the Treble pot is basically off until I turn it that last tiny bit then it OPENS up big time. Guess I just gotta get a new pot huh? Any ideas on the hum? Is there a common issue that causes it?

oh and did I mention the vibrato doesn't work at all?

The first thing you need to do is get your negative bias situation under control. If you've got red-plating you may have already damaged your power tubes.

Take out your power tubes and verify you've got negative voltage on each pin 5 of your power tube sockets, something around -45 to -55 volts or so.

While the power tubes are removed would be a good time to install a 1 ohm resistor from each power tube cathode to ground. With a DMM set to Milli-volts you can clip the leads across each resistor and the millivolts you're reading are the milli-amps your tubes are drawing.

The Duncan tube amp site has a nice little Excel bias calculator you can download. YOu enter the tube type and your plate voltage and it will show you the milliamps your tubes should be drawing.

You really should get yourself a variac and start using it to slowly power up your amp while monitoring the current draw for the power tubes and power supply voltages; flipping the power switch and hoping for the best is not a good practise. You may have already fried your power tubes but maybe not...

Do the above before you report back...

TT

I have a 10 ohm resistor going from the 6L6s to ground but I only have one 10 ohm, is that alright? I backed that bias range resistor down to 47 ohms and now I'm getting manageable currents. I have it set at 35 mA but I'm going to bias it closer to 40 and see how that sounds. I figured out my problem with my vibrato also--forgot to connect to ground where the vibrato switch is supposed to be connected (I just wanted vibrato on all the time). Will connecting that to ground get rid of the hum I'm getting? I figured it very well could. I'll report back after I try those things

Okay, another update. I grounded the vibrato wires but now it just ticks or thumps kind of. doesn't do what it's supposed to. And I'm still getting that hum. Could that be from the power tubes being damaged?

Sorry for posting so many times back to back but I keep experimenting and finding things that might be a better give away of my problem. When I turn the volume knob past 1 or 2 it has a high squeal. When it does that, the mA jump up pretty high.

On a side note, I wanna make sure I'm doin this right: to test plate voltages and what not, I fire the amp up with a dummy load with no tubes connected right? I really haven't tested the voltages running yet because I was still unsure of that and have only found speculation about it on the internet. I'm not confident in the opinions that are posted elsewhere

Just turned off the standy on the amp and it sounded like a siren going up in pitch. I'm hopin by lookin at the board and giving you guys all this stuff, eventually i'll figure something out

Hi chum, sorry to hear of your dispair.
Maybe forget all the preamp stuff you really need to find out what the power stage is doing.
Disconnect the PI from the rest of the Amp by pulling all the preamp tubes, now let the amp warm up read your plate Voltage > read your bias voltage (At the grid of each power tube) Onto reading the the CS voltage, then do the maths.

Side note;
Loss of bias voltage is probably the most common reason for destruction of output stages in fixed bias Amps.

You can use one CS for both tubes but I'd be using 2 seperate CS points thereby allowing you read each Valve.

Blue glow is not good Are the Tubes brand new?

(I was caught out with old tubes once,, had me tearing my hair out.
Old tubes that have lost emission can start wandering the bias a fair bit.)

Once you are happy inject some audio into the PI and see if you get a good clean signal coming from speakers.
If all's well then focus on the rest of the circuit adding bit by bit, otherwise you will get yourself in a mess.

If you are unsure then post the plate voltages and bias along with the CS reading you are getting

Phil.

Okay, first of all, I should tell you I just used regular stereo jacks for the input and just grounded the ground (instead of having fender style jacks). I don't think that's what's causing all this but i figured I better tell you just in case. When I'm testing plate voltage and the rest of testing, I take out the preamp tubes and leave the PI and the power tubes? just making sure I have that right

Just took the voltage readings. pin 3 and 5 measured 505 volts. pin 4 and 8 measured -48 volts. To me, that seems totally wrong. Shouldn't pin 3 and 8, the plate and cathode and beam plates measure 505v and the grids measure -48v?

*EDIT* NEVERMIND, I see it now. So, doesn't this mean that the power tubes are getting exactly the kind of voltages they should be getting?

*EDIT AGAIN* No, I was right the first time right? Like Tic Tac said, grid 1 should have negative voltage? But I have it on grid 2 and pin 8. I'm guess that is where my problem lies huh?

*EDIT YET AGAIN* I was right the second time--i had my pins switched up. Pin 3, plate is 505v with no tubes in. Pin 4, grid 2 is 505v. pin 5 grid 1 is -46v. So, what do I check next?

Just tested voltage for all the preamp tubes also and the PI. Here's what I got:

PI: pin 1 260v (supposed to be 230v) pin 3 102v (supposed to be 106) Pin 6 260v (same as pin 1 I assume?)

first preamp tube: pin 3 3.5v (supposed to be 2.1v) pin 6 230v (supposed to be 270) pin 8 1.61v (supposed to be 2.1v)
Second: pin 6 236v (supposed to be 270)
Third: pin 3 2.2v (supposed to be 2.5) pin 6 390v (RIGHT ON TARGET! ) pin 1 250-260v (supposed to be 280v)

So, are those close enough to not raise suspicion? Or should they be a lot closer than they are? On the fender schematic it says +/- 20% and they all fall within 20% of the suggested voltages so I need your expert advice on what to check next