First Build troubles - 5E3 w/ Voltages & pics!!!

[tubeswell]

Hi Every one,
My Name is Peter, this is my first post on this forum , I have just finnished building 5E3, and I'm also having problem voltage readings.
But I will work on them, For now I was woundering, how do I measure the tube bias for the 6V6gt's , not sure how to do this, I think I might have the tubes rather on the warm side, but need to find out.

Peter

Hi Peter

You need to measure the plate current and the plate voltage for 'proper' biasing. But you can get away with measuring the tube current (instead of the plate current) and subtracting about 2mA (assuming typical 6V6 screen current).

So in a 5E3 you measure the tube current at the cathode(s) of the 6V6s by measuring the DC voltage-to-ground at the 'top' of the cathode resistor. To do this properly, you should know the exact resistance of the cathode resistor (which you will need to measure without having the cathode bypass cap in parallel - i.e.: you will need to unsolder the cap - otherwise you will not get a proper resistance reading. Usually helps to measure the resistor's exact resistance before you assemble the resistor into the amp).

Then you divide the cathode voltage by the cathode resistor's (actual measured) resistance, to get the current through the cathode resistor - which, in a 5E3, is the total tube current for both 6V6s* (according to ohm's law: E = I x R), so you will need to divide that by 2 to get the tube current for each 6V6. Then you subtract about 2mA (assumed screen current) from the tube current for each tube, to get the (assumed) plate current. (say you get 80mA through the resistor - that's 40mA per tube, and about 38mA plate current per tube)

* because the cathode resistor is shared by both tubes

Then you measure the plate-to-cathode (DC) idle voltage of each 6V6. Then you multiply the plate-to-cathode voltage by the (assumed) plate current, and you get the plate dissipation (in Watts) (i.e.: V x A = W). (say you get 340 plate to cathode voltage, that's 340 x 38mA = 12.92W)

For 'proper' Class AB1 operation in a PP amp, the plate dissipation should be about 70% of the tube's maximum rated plate dissipation. A 6V6 is rated at 12W (so doing it 'properly', you want about 8.4W per tube). But since a 5E3 is cathode-biased, you can get away with the tubes running at full blast.

[loupy31]

Hi Guys,
I measured the bias ma and came up with 45.5ma and 45.2 ma on the 6V6GT's
Is this to high if so what is required to get a good reading

Peter

[d95err]

Hi Guys,
I measured the bias ma and came up with 45.5ma and 45.2 ma on the 6V6GT's
Is this to high if so what is required to get a good reading

Peter

The current (mA) means nothing without the anode->cathode voltage. Measure this and do the math above to calculate the plate dissipation. Note: Measure the voltage from anode to cathode, not to ground.

[loupy31]

which pins are the anode and cathode

[Wittgenstein]

Are you getting any hum? It looks like your heaters are pretty close to the leads to the tubes. If you aren't getting any heater noise then no worries. If you are, try tucking those up against the outer lip of the chassis, away from the other wires. That might help cut some noise.

I do have some hum and it is really getting to me. The 5F1 I built is whisper quiet, unlike this build. I am suspicious of the grounding though, because the hum is greatly reduced when I touch anywhere on the chassis or jacks. It does not completely quiet the amp when I touch it. It sounds like there is still a small amount of 60 cycle hum. I have was messing around with this more last night to try and pin it down. All the ground wires look good and show continuity to chassis. I removed the PT's heater center tap and added an artificial tap using 100Ohm resistors raised to the 6V6's cathode voltage, but this did not seem to help at all. I also tried the HT center tap at both the star ground and first filter cap negative, with no difference.

When I had the amp on I tried moving the heaters around to see if they were inducing any noise. It did not seem to matter where they are located (under the lip, or where they are now). I'm wondering if I should go back and isolate all my input and output jacks to run ground wires, like I did in my champ build. Most advice I revived on this amp however, said this was not necessarily.

I have already built and rebuilt this circuit 3 plus times now, so I am not too worried about making more changes if it means a quite amp. =)

[ThermionicScott]

Shouldn't have to do fancy grounding to get a quiet amp. Have you tried pulling preamp tubes to isolate which stage is producing the hum?

- Scott

[Wittgenstein]

You know... I have not tried that since the last revision. Before however, I know it was the first pre-amp tube causing the troubles. When I pulled the 12ay7 it went away. I will definitely give that a try again tonight when I get home and post the results.

[loupy31]

Hi All,
Can someone please tell me how I can do my voltage checks and how to do my bias checks or maybe give me a link to a site that can explain it to a noob like my self.

Peter

[Wittgenstein]

I was thinking I might try connecting the 68K RC resistors directly to the the 12ay7's pins, and then run the shielded wire back to the input jacks. Currently I have the resistors attached directly to the input jacks. It will be a good opportunity for me to shorten my shielded grid wires a bit. More to come....

[Bruce / Mission Amps]

Who is Revolver Amplification?
I would say that if you think the Hoffman ground scheme is or was a "HUGE" improvement over the tried and true Fender one, then you probably didn't have it wired right in the first place.
Also, if wired correctly and carefully, there really is no need for shielded cable in the amp anywhere and only a very small improvement by using it from the volume pots to the 12AX7 #2 lug grid.
Yes, some builders short the grounded shielded cable the center conductor or somewhere else when soldering, check that out. But if it is grounded at lug #2... there will be virtually no audio... you say you have some audio so I'd be looking elsewhere too....
Look to see if you have your phase inverter wired correctly.... look at where the 1k5 resistor goes and the 1M resistor with respect to the 56K resistor. ....

Added:
Check the voltage on lugs 7 and 8 of your 12AX7.... that doesn't look right from on earlier post.

[Wittgenstein]

My 12A125-A 8-Ohm 30-Watt speaker and lacquered cab arrived today despite 6+ inches of snow. Luckily my chromed chassis from Tubes+More fit the Weber chassis and pre-drilled holes. Now that the chassis is mounted in the cab I have noticed some bad tones (sounds like a broken speaker more than rattle) when I play certain notes, which I think is being caused by a microphonic tube. I will have to try and swap em out one at a time and see if it makes a difference. Here are a few pics, my les paul copy is currently being upgraded with new electronics:

[mfreqmaster]

How do you like the sound of the 12A125? Compared to the previous speaker you were using? I have a 5E3 build with a Jensen P25R, & have read that these newer speakers don't sound the same as the original. I was thinking of trying a Weber in my cab, but am not sure yet.

[Wittgenstein]

How do you like the sound of the 12A125? Compared to the previous speaker you were using? I have a 5E3 build with a Jensen P25R, & have read that these newer speakers don't sound the same as the original. I was thinking of trying a Weber in my cab, but am not sure yet.

Well, I was using the cabinet and speakers from a Peavy Classic 50 (2 x 12"). I tired the speakers wired in both parallel and series, but overall it did not sound good to my ears. From what I have read they are not terrible speakers for the Classic 50, but they were never meant for a 5E3. So I did notice a big improvement from those, however I have not had much of a chance to really play on my new speaker, and it has yet to be broken in to give it a fair comparison. I will try to give it a chance to break in a bit and then upload some clips of the difference of the two cabs.


Edit: FWIW, I was very pleased with the service from Weber and would recommend it in the future. I forgot to add that I wanted my cab aged and he added it on free of charge since I ordered the speaker as well. It shipped in less than 2 weeks. I paid $355 for my cab and speaker shipped.

[tubeswell]

Weber Speakers are made the old-school way and they need a little break in time. They sound a wee bit harsh straight out of the box, but get a lot better with a few hours playing IMHO

[Randall]

I have a 12A125 in my 5E3, and I like it better than the made in Italy Jensen in my friend's Victoria.