No amplification from a 6V6

[IDrinkAlone]

Hi all,

I have a question (maybe a weird one, I'm just a hobbyist so forgive my ignorance)

What is the approximate AC Signal voltage I can expect on the plate of a 6V6 ?

Here's a short story behind that question. I'm currently building a guitar amp from an old RCA A-25 radio chassis. I'm using 2 6SL7, 1 for the preamp and the other for the tremolo and a 6V6 as power amp. The power supply / output transformer / speaker are from the original radio.

I'm measuring 275vdc on the plate, 238vdc on the screen grid 21vdc on the control grid an a plate current of 43mA

I currently have a strong signal on the grid (pin 5) of the 6V6 when I inject a 1Khz 300mV at the input jack but I can barely hear it with the volume to max. When I check the AC voltage on the plate I only have 305mv hence my question.

Anyone have an idea ?

Thanks
Al

 

[thorpej]

Can you share your schematic?

 

[RWood]

Also post the cathode voltage. If it is grounded and the voltage is zero, that control grid needs to be negative voltage.

If there is a cathode resistor, the control grid should be at or close to 0.

As previous poster said, a schematic will help us help you.

 

[IDrinkAlone]

I don't really have a schematic but here's a ltspice print screen, also I made a mistake in my previous post it was not 21vdc on the control grid but 21vac and I have 10.8vdc on the cathode and 0vdc on the control grid



Thanks
Al

 

[RWood]

That seems right. What happens when you inject the signal into the grid of the power tube? And into the grid of the previous stage?

i assume you have a speaker in place of R3.

You might ohm out the OT to check for continuity as the circuit itself looks okay to me. Others will chime in.

 

[gadget73]

honestly sounds like shorted turns on the output transformer, or a short on the load side to me. Its apparently not open circuit since you have plate voltage, but it acts like an AC short. You have bias and grid drive in the range I'd expect so the tube and the driver seems to be working.

 

[IDrinkAlone]

From the original A-25 radio schematic the OT is supposed to have 450 ohms of DC resistance on the primary side and the speaker is 3 ohm. I just checked and on the primary side I have 500 ohms which is fine, the speaker coil show 2.6 ohms of dc resistance which look fine too but on the secondary of the OT I have 0.2 ohms of dc resistance which seems to be too low...

What do yo think ? is the secondary short ? is there a way I can validate this or the 0.2 ohms itself is enough to conclude it's short.

Thanks
Al

Edit:
I just checked the impedance with a 1khz signal on the primary and it give me 7k ohms so I think the OT is fine

 

[IDrinkAlone]

Are you 100% sure you have the 6V6 wired correctly and pins identified? Having 21v signal ( from meter as RMS) on the grid is 30v of peak signal driving a cathode voltage of @10v. That's way over anything normal on the grid. It's easy to get the pins miscounted backwards.

That 21v is at max volume, I also checked on the plate of the 6sl7 (previous stage) and I have the same thing. At that level I have a signal +/- 300mv on the plate, if I turn down the volume to have signal on the grid of around 6v then the signal on the plate go down to around 200mv.

I don't have problem with pins identification and it "seems" wired correctly but I I'll have to go through each connections to make sure that they are solder correctly and wired correctly (I'm really good at making stupid mistake)

What approximate signal voltage should I expect on the grid & plate ?

Thanks
Al

 

[IDrinkAlone]

OK, one other issue with reading the big AC signal on the plate is that you need to use the B+ DC going to the transformer as the (-) probe instead of the chassis ground since the AC is riding on top. It has to reference the DC swing because the signal is not true AC that goes (-) ever.

The grid signal (voltage peak, not RMS) should be less than the cathode voltage... 11v cathode is normal.

Ok thanks, I'll have a look at the grid signal, for the plate it's not a big signal it's +/- 300mV so is it normal that the plate signal is much lower than the grid ?

 

[IDrinkAlone]

The plate voltage will normally swing by 75% or more. But in order to see it on the meter as AC you have to put the common probe on the steady B+ and use AC mode.

The plate voltage will normally swing by 75% or more. But in order to see it on the meter as AC you have to put the common probe on the steady B+ and use AC mode.

I'm currently looking at it with a scope with AC coupling on to remove the DC component and only see 300mV at full throttle (and I barely hear it on the speaker)

I think I'll take a break for today, tomorrow I'll check all connections and I'll go from there.

Thanks
Al.

 

[gadget73]

if you want to check the output transformer, connect the leads to some available AC source with limited current capability. The 6.3v heater supply with a 100 ohm resistor in series with the transformer will work. If the transformer has a short, the 100 ohm resistor will get warm and you will get no AC on the secondary. If its good you should measure some amount of AC on the secondary side of the transformer.

If you can give it enough voltage to get close to 1vac on the secondary side, you can confirm the winding ratio on the transformer. primary volts / secondary volts = winding ratio. Winding ratio squared x secondary load = primary impedance. I'd guess somewhere around 5K PI if its typical datasheet specs.

Do not attempt the higher voltage test with a shorted transformer. Things will end badly.

 

[IDrinkAlone]

Ok I found the issue, like I said in a previous message "I'm really good at making stupid mistake" and it was one of them.

In the print screen I shared there was no power supply section (it was easier to just put some voltage source in ltspice)
but in real life I did put one more filter cap just to make sure to clean any ripple off the 6V6 plate so it was looking like that


and it was doing a good job filtering ...

Now just for my understanding, is it because that cap (with the plate resistance) create a low pass filter ?

Now I still have some tweak here and there but at least I have a strong signal to work with

And a big thanks to all people that took time to respond.
Al

 

[trainbuftony]

Cant have a cap there. It will slurp up all of your music as if it were 60 cycle ac.
The last ps cap for the output has got to be on the opposite end of the opt coil from the tube. The way you have it configured your driver is swinging some voltage to your power tube, but your power tube isnt drawing power through the opt coil its drawing most of its (music) power from that cap. The current from your power supply, charging the cap through the output, is responsible for whatever you get at the spraker terminal which i assume is very distorted.
Ditch that cap, and it will probably work much better

 

[gadget73]

yeah, the cap was acting as an AC short to ground. That would absolutely explain what it was doing.