4-4-0 [2 NEW BUILDS WITH PICS; THE 2 WATT WRECK]

[Cliff Schecht]

Did you remember to install a fuse? I've done that on more than a few new amp fire-ups.

[amplifiednation]

yes and i have power at the switch but it doesn't seem to be going into the PT???

no power at the heaters or anywhere...i wonder if i wired the PT incorrectly?

[Cliff Schecht]

FWIW I always start my testing and this type of troubleshooting with a low voltage function generator. Set it to a 60 Hz sine wave and max out the amplitude. Even after the transformer the current is so low that you won't be getting shocked. This makes it much easier to probe around on a power section than having to worry about getting hit by 120V or more.

I see an issue I think though. From what I can tell in the picture it looks like you wired the IEC connectors hot and cold terminals in parallel with the fuse. This is wrong, the fuse goes in series with only the hot side of the primary. Putting it in parallel like that will never work.. It would instantly pop the fuse that is essentially shorting hot to cold when power is applied.. Since it looks like you did something similar, study how the Trainwreck Express power supply is wired up more carefully.

[amplifiednation]

yup....that was a bonehead move. i'll get the hot wire going through there and see if i can complete the circuit!

[amplifiednation]

So i rewired that fuse so the hot wire goes through it and the neutral goes right to the switch. Definitely a rookie mistake.

So I'm getting some power out of the PT now, but the only place I'm finding significant voltage is where the diodes connect to the middle turret. I'm getting well over 300 volts DC. When I flip the standby switch it disappears.

I have nothing for filaments. I check continuity and it's there. Any ideas? No AC even before the resistors.

I am not blowing fuses and got no activity out of the bulb limiter at all...no flash.

[Cliff Schecht]

I don't think I see a ground reference on your heaters (either center tap or 100 Ohm resistors to ground).. With all of the tubes taken out does the heater taps of the power transformer show any AC voltage (measure from green to green)? How about from red to red? Does 120V appear when you measure from hot to cold?

Again a function generator can make all of this testing MUCH safer. You run the output of the function generator into the hot and cold on your wall plug and all of the secondaries should show a consistent, measurable voltage (you can calculate the voltage ratios and expected voltages easily as well). When you flip the standby switch on you can even measure appreciable DC on all of the B+ nodes to make sure that you're getting voltage all throughout the amp.

[NickC]

........... a function generator can make all of this testing MUCH safer. You run the output of the function generator into the hot and cold on your wall plug and all of the secondaries should show a consistent, measurable voltage (you can calculate the voltage ratios and expected voltages easily as well). When you flip the standby switch on you can even measure appreciable DC on all of the B+ nodes to make sure that you're getting voltage all throughout the amp.

Greetings Cliff,

What make/model/features do you recommend that might be had on eBay for reasonable cost? Thanks for any insights you can share.

Best Wishes,
Nick

[amplifiednation]

Cliff. Thank you for your help. Unfortunately my test equipment is bare bones. I have a mmv and a home made bulb limiter!!! haha, i'll get there.

I'm going to start a new post with my troubleshooting issues instead of hijacking this one.

[Cliff Schecht]

For this kind of testing (i.e. testing transformers) all you really need is a function generator that can go down to 60 Hz with some appreciable amplitude. It's also nice to be able to shrink that amplitude and increase the frequency to 1 kHz for signal path testing (i.e. making guitar-like signals). You can clip a function generator to any input in the circuit to test if it's making noise.

A scope is also a nice tool to have, actually very nice once you know how to really use one. It can help you track down RF oscillations (and cause them!), where signal is dying, where signal is clipping (not enough or too much), can give you frequency information, etc.. The fancier ones can even do FFT's, screenshots, averaging, fully differential signals (phase inverters) and lots of other neat things.

For a good cheap function generator on eBay, look at the old HP 331x series stuff. The 3310 is the cheapest usually and I prefer the beefier knobs compared to the 3311 (take a look at the pics yourself) but either one will get the job done cheaply. If you can find a digitally programmable 33120 for cheap then look there but these are going to be fairly $$$ and overkill for this type of testing. FWIW I used a 33120 last to help develop a 250 nV test signal for a medical device I designed that amplifies up to (and above) 140 dB. Like I said, overkill for guitar amp testing..

[NickC]

^^^ Thanks Cliff, much appreciated!

[mehfuzhoss]

I am finishing off this amp. I would like to know to how to bias
The two trim pots.

Thanks.

[pompeiisneaks]

I'm not sure which build this is exactly, since it's talking about 2 new builds and is a quite old dead thread (you usually should start a new one linking back to this one or just to the schematic and layout of the amp you're building) but effectively to use dual bias pots, you check the bias of each tube and use the bias pot for the power tube it's connected to, to bias the tube according to the right calculations of the anode B+ and the tube type. Once you know your 70% dissipation rating expected mA, you calculate the existing and dial it up or down to achieve that.

If there was a bit more information, I'd be able to be more precise (basically I don't feel like reading through an 18 page thread to find out the details mentioned above)

~Phil

[sluckey]

I am finishing off this amp. I would like to know to how to bias
The two trim pots.

Thanks.

The biasing info is on the schematic.

[romberg]

If you look at a 6sn7gtb datasheet:

http://www.tungsol.com/specs/6sn7gtb-tung-sol.pdf

You will notice that it lists a maximum dissipation of 5w per plate. But... It also lists 7.5w maximum dissipation for both plates.

Since the 4-4-0 is using both plates, you will want to *not* use the 5w rating but instead go with 7.5w for both which is 3.75w per triode.

So, assuming the 4-4-0 has about 400V on the plates (it has been a while since I measured mine but I think this is close) you would want
to adjust the bias until you read around (3.75W * 0.7) / 400V) = 6.5mA of current on each triode.

Mike

[mehfuzhoss]

Okay. So i will put my red and blk probe and trim it untill
It reads 6.5 ma on the meter and repeat the same for the other trim pot

Am i right?

Thanks