Problem with startup

[jckid649]

xtian says it above... the bias trimmer is wired incorrectly. The center and one outside leg should be shorted together, not the two outside legs.

Thanks

[jckid649]

So what is your voltage at pin 5 ?

With no tubes in at pin 5 of V4 or V5 I was reading around 24.7ma. So I knew something was waaaaay off.

[rdjones]

So what is your voltage at pin 5 ?

With no tubes in at pin 5 of V4 or V5 I was reading around 24.7ma. So I knew something was waaaaay off.

You should be reading voltage here, not current.
And it should be negative voltage.

rd

[jckid649]

Im sorry. I meant to put the -in front.

I reinstalled the trim pot properly my max reading at pin 5 is now -35.7ma without the tubes.

[Teleguy61]

Please build and use a light bulb limiter for startups.
Saves fuses, tubes, transformers, and gray hair.
Search here or online--very simple.

[Bruciep07]

Im sorry. I meant to put the -in front.

I reinstalled the trim pot properly my max reading at pin 5 is now -35.7ma without the tubes.

I'm going to make the assumption that you skipped over rdjones' comment about measuring voltage, not current. I'm also going to make the assumption that you wrote -35.7mA but meant -35.7V. If, however, you are measuring current at the bias point purposely, you need to take a huge step back and do some research on why you're doing what you're doing. Don't take this as a shot against you, but some basic understanding of electrical circuits/how vacuum tubes operate is needed before you should be tackling builds like this. I realize the "build guide" provides a sort of "paint by numbers" guide that allows just about anyone to successfully put together an express. I feel like I need to beat this dead horse, but there are lethal voltages/currents in these amps, putting together an amp from pictures with little electrical knowledge is a dangerous combination. Again, I don't mean this as a knock, not trying to derail your build, if anything TAG is the place to come to learn the craft and get help from talented builders. But pace yourself, we all started somewhere, learn what is going on and why, it's much more rewarding to be able to troubleshoot and fix your own mistakes than copy a picture. Important rules to live by at/before startup.
1)No tubes before checking voltages
2)lightbulb limiter (or/and a variac) is the "safe" way to go at initial startup.
3)one hand behind your back when measuring current/voltage, the last thing you want is a few amps at 350V across your heart.

Again, this is a great community for people just starting out, pace yourself and learn from guys here before you jump in with 2 wet feet...

bp

[jckid649]

Yeah i realize alot of this is above my paygrade but I'm in this too deep now and it's do or die.

I'm trying to just follow the startup guide but I guess I just don't really know what I'm looking for wrong. I did use the bulb limiter and the guide said that in standby with no tubes when you flip the power switch on it should do nothing. Then it should flash and then dim down to a subtle orange glow when flipping the standby off. However, mine flashes and dims when i flip the power switch on with the amp in standby....so idk.

[vibratoking]

Yeah i realize alot of this is above my paygrade but I'm in this too deep now and it's do or die.

Please make sure it's do and not die. As mentioned, there are lethal voltages and currents in there. I would rather see you fail at the repair then fail at safety. If you aren't 100% certain, please don't proceed any further.

[jckid649]

Yeah i realize alot of this is above my paygrade but I'm in this too deep now and it's do or die.

Please make sure it's do and not die. As mentioned, there are lethal voltages and currents in there. I would rather see you fail at the repair then fail at safety. If you aren't 100% certain, please don't proceed any further.

I do appreciate the concern. I have learned enough to know what not to touch and how to test things in the circuit. I'm just not very good with the technical jargon or how to read schematics. I'm trying to Skype with a friend in CA who is a builder and he's trying to talk me through testing. It's just a slow process cause there's a lot of the terms I don't understand.

[jckid649]

After tons of wiring reviews and several errors discovered and corrected most of the readings came up to spec... however, bias is still reading a little too hot at max setting with a C- ranging from -23 to -37 from one end of the bias trim setting to the other...may need to change the 47k resistor(?)...after skyping with a tech friend in California i finally now have everything straight with the wiring and readings for the most part...used the bulb limiter in the testing also. However, now having reached the point of having tubes in and coming out of standby i have an oscillation/squeal issue.

after several sound experiments the sound can be an oscillation or just a high pitched squeal based on its positional relationship to the speaker cabinet...it squeals with the volume and all controls all the way down and volume on zero and with or without a guitar plugged in...when the guitar is plugged in and you hit the strings it goes away till the sound of guitar fades away then it starts squealing again.

I suspect an input issue at V1(?)

[Firestorm]

Try swapping your OT primaries (or secondaries). Oops. Secondaries are multi-tapped. Swap primaries.

[jckid649]

Try swapping your OT primaries (or secondaries). Oops. Secondaries are multi-tapped. Swap primaries.

This may be a dumb question but I had another tech friend suggest the same thing but I insisted that I've checked and quadruple checked those primaries against RJs wiring diagram and compared them to the info in the build guide but I know anything can happen righr. However, I just gotta ask this.

If I wired the heaters to the preamps and power tubes out of phase would that require me to wire the primaries the opposite way or does that matter?

[M Fowler]

Just swap the OT primary wires as suggested it's out of phase.

You need to stop building by layout only, wire color, etc. Use the layout as a visual helper but rely on the schematic for actual connections.

[Firestorm]

Not the heaters that are at issue. It sounds like the feedback loop is producing positive rather than negative feedback.

[martin manning]

Not the heaters that are at issue. It sounds like the feedback loop is producing positive rather than negative feedback.

...or not. The oscillation could be unrelated to the feedback loop, but it's very easy to swap the leads and find out, or just disconnect the feedback.