300T...late '90's...

[Wally]

HEllo, everybody. I have a 300T head sitting here waiting for power tubes to come in. I am simply wanting to know if anyone has any insight and/or warnings about this amp. I don't see many Sunns around here, os if there is any heads-ups I would appreciate it.
One thing the owner is concerned about is the biasing. This amp has been reading 81mv on the back panel since he got it about a year ago. HE was concerned because the factory suggestion is to run the amp biased at 100mv. I told him that as long as he liked the sound, that 81mv setting would simply be a cooler setting and his power tubes woudl last longer.
I may well set the bias at 100mv if the new tubes run in such a manner as to allow the bias range to reach that setting.
Any pros or cons here? IF the bias circuti won't allow that setting, should I change a bias resistor to get intothat range?
I suppose all of this depends on plate voltage, which I have not yet measured. I will have it on the bench by the end of the week since I didn't have a matched set of 6550C's lying around.
Any hlep or insight is appreciated.

[Wally]

Well, I got the power tubes in and have started looking into this amp. The preamp tubes and the PI are noted to be 12AX7's on the chassis. What I found in the first two positions in the preamp were two 9AU7 tubes. From a quick investigation, these tubes seem to be similar to 12AU7's in the gain category. THeir heater voltage requirements are different being 4.5V/9V.
Does anyone have any insight/thoughts on this...why, is it safe, any thoughts at all. Any thoughts are appreciated.

[Isaac]

Wish I could help, but I don't know enough about tube electronics to do so. Others here know much more.

[mckinnon audio]

  Hi there,either someone dropped the htr. volts from 12 to 9 with a few resistors or the htr. elements are tough enough to take the 30% over voltage.I'd measure the htr. volts on the tube sockets with the tubes removed to see what it really is.If it's 12 volts,I'd replace the 9AU7A's.Good luck,Mel.

[Oli]

Hi,

maybe U got it already:

http://www.mif.pg.gda.pl/homepages/tom/schematics.htm

Greetings

Oliver

[Johnny]

On my 300t I noticed the bias dropped as the tubes wore, I was able to re-bias it to 100,  but replaced the tubes anyway.

The PI in mine and in the schematic is a 12AT7. It's a good idea to replace it when replacing power tubes.

JJ ECC83's sound great in V1, 2, and 3. You'll have to remove the head from the cab to see of it was modified for the 9AU7's.

While you have it out look at where the heater wires attach to the power section. These overheated and burned out on mine.

These are great bass amps especially when used with the 215B cabinet.

[Wally]

My apologies for my late response to your replies.
HEre is what I found. The first two preamps are DC heated at 12 volts. The 9AU7's in there are holding up. This owner has had the amp for over a year and was running it as he bought it.
WE did some listening tests and decided to run the 9AU7 in the first position, which is the clean channel preamp. WE ran a 12AX7 in the second position. IT provided better gain for the gain/volume 'OD" channel. HE is happy with the results. I suppose these 9AU7's are tough tubes regarding their heater filaments. who knows who and why these were put in there. I called a fellow in Dallas who sometime in the past left his repair label in the chassis....no response from him. IF and when the 9AU7 fails, he has a spare 9AU7 or a new 12AX7 to putin there, and I will have a 12AU7 for him if he wants the reduced gain for that channel..
Thanks for all of your responses.

[loudthud]

In the 300T the heaters for V1 and V2 are connected in series (12V mode pins 4 and 5) and there is a series resistor, three 100ohm in parallel. This all runs from the regulated +/- 16V supplies. Was the resistor changed? With the standard resistor(s)  12AX7s would see roughly 13.35V and a 9AU7s would see 11.44V assuming the heaters have a linear tempco which they don't. Because as they warmup the resistance gets higher, the 9AU7s are probably just 10-15% over voltage.

[Wally]

loudthud, thanks for the info. When I next have the amp available, I will check all of that.
Here's what I found while I was checking voltages. All tubes except V1 and V2 showed proper AC voltages. In AC mode, V1 and V2 did not give me the expected readings. Because the tubes were obviously getting heater voltage, I switched the meter to DC and found 'normal' DC readings....approximately
6.2VDC on each leg. Should these two sockets be provided with DC...or has someone rectified (?through some diode arrangement?) the voltage to reduce hum? When I was in the amp, I didn't have the schematic that was kindly provided above. THe player was headed out on the road, the amp had been operating in this situation for at least the year he has played it, and I let it go as is....even with the 9AU7 in V1....once again, since it has been operating there for at least the last year.
I will study the schematic above. Yes, the sockets were wired with pins 4&5 together ...as I am used to seeing; but I didn't take note of the circuit beyond that since the heater filaments were receiving good voltage...albeit it in DC form.