6V6 Rocket build low output

[valveandsound]

Greetings everyone!

I have been building a 6V6 powered Rocket with the classictone express PT and OT and I am kinda stumped on a low power issue.

The build is complete, and I have audited it against the taylor rocket schematic so I know my connections are correct. Other than 6V6 setup the other small changes I made were installing a PPIMV, 47uF mains filtering instead of 80uF, and added a 500pF coupling cap that toggles in parallel with the 500pF cap on the first stage to fatten the preamp up a little bit. My preamp and PI voltages are well within 10% spec based on the Rocket voltage chart. My plate voltages on the power tubes are 387v, screens are 380v and being a cathode biased setup it idles at roughly 10W/tube dissipation.

So, with all that being said the amp sounds like a rocket but the output seems quite low. It doesn't appear to break up early, just nowhere near loud enough. So i run a 600hz input signal and measure output across a dummy load. I am more or less getting 5W of output before clipping. I have tried swapping the OT plate wires (which was probably pointless; the rocket doesnt have NFB lol) with no change. I have also clipped in another output transformer with a 6.6K primary and measured that across a dummy load as well, no difference.

I have also tried inserting the signal at the phase inverter input with no real change; so to me that rules out the preamp. scoping the output of the phase inverter, i dont see any odd clipping - I get about 15vAC of signal going to the power tubes before clipping. I'm not sure if that is an obvious red flag or insufficient drive voltage for the 6V6s.

Any thoughts from the collective wisdom here? Thanks in advance.

[pompeiisneaks]

That seems a bit low, like you've got a master volume attenuating. It seems to me you should usually get about 40 or 50 VAC of sine input before it even starts clipping as I recall, at the PI output side (in general, from my experience, not from a specific rocket build). Sounds like something is starving the signal in the chain. Seems you have a sig gen, do you have a scope? If not putting your DMM on ac volts will help, just look to see that in the first stage you have about 400 to 500mV input from the sig gen, and then record the output post the coupling caps of each stage and report back. take some pictures of the guts too, some of the more seasoned builders may see something obviously wrong.

~Phil

[dbeasley]

Thanks Phil. My gut was telling me that I should have much higher output from the PI. I might bypass that PPIMV and see what's up.

That seems a bit low, like you've got a master volume attenuating. It seems to me you should usually get about 40 or 50 VAC of sine input before it even starts clipping as I recall, at the PI output side (in general, from my experience, not from a specific rocket build). Sounds like something is starving the signal in the chain. Seems you have a sig gen, do you have a scope? If not putting your DMM on ac volts will help, just look to see that in the first stage you have about 400 to 500mV input from the sig gen, and then record the output post the coupling caps of each stage and report back. take some pictures of the guts too, some of the more seasoned builders may see something obviously wrong.

~Phil

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[Littlewyan]

A quick check is to measure resistance across your master volume. Measure it from the PI side to ground and then the power tube side to ground, if that makes sense. It should read 220K with the volume all the way up, on both sides. Also double check your resistor values in the PI circuit. Sometimes we get a few resistors mixed up and end up with very low output from the PI.

[valveandsound]

I ended up pulling the PPIMV setup and trying again - I still only get 7 watts of power. And that is with injecting a signal into the PI input, ruling out the preamp IMO.

Voltages in the PI are close: Grid connections are 33v/35v. PI plates are 200v/189v. Cathode voltage is 43v.

Voltages on my 6V6 tubes are 400v/plates 395/screens.

using a 500R/220uF cathode setup i get roughly 30v across the cathode resistor, which by my math nets me 94% idle dissipation which for a cathode bias amp seems to be fine.

I double checked all components in the PI - all resistors and caps are well within spec values. The voltages in the PI reinforce this.

What the hell is going on??.

[pompeiisneaks]

Did you double check that you're getting that 33/35VAC on the grids of the power tubes too to rule out some loss in transit? Also a photo of the output jacks and power tubes may help us see if we note something amiss. I just found a major problem on mine after I posted my photo and blew it up. I found a solder blob that had dripped down on the jack touching between the tip and ground. I couldn't see it with my shit eyes until I blew up the photo and was like WOW! That's obvious!

~Phil

[Littlewyan]

How are you measuring the output power? Just because there's a few ways of doing it

[valveandsound]

I am getting about 15v signal drive to the power tubes, and thats after the master volume has been eliminated.

Wyan, I am calculating amp power by running the amp into a dummy load and scoping the output across the dummy load. I am running a 600hz tone into the PI and into the front end of the amp - raising the signal until the tops and bottoms of the waveform start to just slightly clip. From there, I square the output voltage (11.2v X 11.2) and divide it by the load resistance (16R) to get roughly 7.8 watts.

Also, for grins I used my X100 probe and scoped the plate connections on the tube sockets. From that point, the signal looks similar to what it looks like going into the power tubes (as far as waveform). I get roughly 140vAC signal off the plates going into the primary of the output transformer.

I double checked all my connections on my power tube sockets, PI connections, and speaker output jacks. I dont detect any wierd solder blobs or bridged/bad connections.

[valveandsound]

So, checking signal voltages elsewhere, this is what I get:

clean max signal from the first gain stage taken at the top of the volume pot: roughly 7vAC

Signal taken from the PI input cap clean: 4vAC

Do these seem in line or very low?

[Littlewyan]

Try using your multimeter to measure the output to see if you get a different reading.

Also measure the HT voltage when the amp is under load to see how much it sags. Do this all the way along the HT line.

And put up some pics if you can please

[dbeasley]

I will post pics here in a bit. I appreciate everyone's thoughts so far!

So the wierd thing is I strapped my multimeter across the dummy load and get 24vAC, whereas my scope is showing me roughly 11vAC.

24vAC in a 16 ohm load is roughly 36 watts. I'm thinking there is no way I'm getting 36 watts out of a pair of 6V6s running at 400v!

I'm using a Fluke 117 meter - it's a true RMS meter. I'm wondering if the AC voltage reading is wrong because it's measuring a 600hz tone?

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[Littlewyan]

Your username has just changed? Really threw me there for a sec!

Is that 36W with clipping? If so that's about right. And try a 1Khz tone. That's what I normally use and what companies tend to use I think.

I must admit I'm not all up together with scopes. I can use mine to look at signals and also use the signal generator but I'm far from an advanced user!!

[dbeasley]

Sorry yeah. I forgot a password for my account and made a new one.

Then made an account for Tapatalk on my phone and forgot my new account password so yeah. Lol

That's 36W at Max clean sine wave. I will double check at 1khz

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[dbeasley]

Ok, so I had a bad multimeter lead. Replacing that gets me the same voltage measurement as my scope gives me, which is 10vAC signal output across a 16ohm load at both 600hz and 1Khz.

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[dbeasley]

Also, measuring voltage change at the power tube plates under load I only get a few volts DC of change.

I've also swapped the PI tube and power tubes from a good known working amp with no change.


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