Princeton Semi-Clone Output X-Fmr?

[Rob's Radio-Active]

Thanks...yeah, it wipes me out.

Cheers,
Rob

[The Dude]

Yes, Rob...... best wishes with your fight.

[Rob's Radio-Active]

Thank you. I will get better.

Cheers,
Rob

[Rob's Radio-Active]

Well, I messed around and it ended up that upsizing the output tranformer a little bit really did do the trick to make the guy happy. I guess that's what matters.
The speaker, a 12" Celestion Seventy 80, should be here shortly.

Cheers,
Rob

It's coming along...

[Rob's Radio-Active]

It's done, finally, except for the new speaker has a rattle-buzz...especially around 200Hz. I will take some pictures and post them. I don't have updated ones just yet.
I took the speaker out and ran it from one channel of a stereo amp driven by my tone generator, and it is the same--so it's not something rattling in the guitar amp or cabinet.
Sweetwater is sending out a new speaker. The rest is built though. Thanks for all the input you all. Again, pictures as soon as I take some in good light.

Rob

[Rob's Radio-Active]

Here it is, less the new speaker.
I tried to make it easy to service. Everything can be accessed under the top, back panel. I included test points for cathode current on the 6V6 tubes.
Any DC mV measured on the test points to ground directly indicates cathode current in mA across their 1% tolerance, 1 ohm resistors.

The "extra" controls/circuits are DRIVE, BIAS, and FEEDBACK.
Drive is pretty obvious--it allows mild to severe clipping at low, or any, volume levels, or clean play through.
Bias allows warmer or cooler tone within normal limits of 6V6 idle current choices.
Feedback simply controls the amount of negative feedback for either a "looser" livelier response or more of a Hi-Fi type low, midrange, and treble relationship.

(The keen eye will see that I flipped the control decaling over the other way. The owner-to-be hates reading upsidedown.)

Cheers
Rob

[Mick Bailey]

What's the chassis made from? Looks to be a decent thickness.

Also, how did you do the decal?

[Rob's Radio-Active]

What's the chassis made from? Looks to be a decent thickness.

Also, how did you do the decal?

The chassis is just a blank aluminum chassis I bought...no holes, etc.
The decals are waterslide type for inkjet printers. Laquer clearcoated over.

Cheers,
Rob

PS: I think I have the schematic finalized now. There may still be errors, but I think I have it cleared up:

[pdf64]

Same comments as post #7, regarding the input impedance and gain staging; just in case the drawbacks of the lower than normal input impedance are not apparaent, they include that the instrument will have a different tonal balance when plugged directly into the amp, compared to plugging into eg a buffered tuner, followed by the amp. Also the instruments volume and tone controls will react differently, eg the volume control will have a steeper taper, see http://www.geofex.com/Article_Folder...s/potscret.htm

I think that the pilot lamp, as drawn, may mess up the virtual CT, and also be rather dim.

For this application, I don't perceive the benefit of the 25k HT bleed / load regulation resistor? Some degree of signal level modulation f tube operating points is often perceived to add tonal complxity 'swirl' to the output A Glossary of Common Amplifier Terms

Current limiting / stopper resistors on the screen grids may be beneficial, eg promote longer tube life when the power amp is subject to being overdriven.

[Rob's Radio-Active]

Thanks for the comments, pdf64. Do you suggest a 1 meg input resistance?
The 25K is just to drain the power supply capacitors for safety. I considered grid stoppers on the power tubes, but since Fender didn't use them in the basic design I was working with, I left them out. The virtual center tap did not affect the pilot lamp, but it has been removed anyway. There was more hum with it than with grounding one side of the 6.3VAC circuit. I don't know why. I had quite a problem with hum from socket 1 (V1) until I removed the virtual CT and replaced the JJ ECC83 with a Tung Sol "reissue" 12AX7.

Cheers,
Rob

[ThermionicScott]

Where's the 25k resistor you guys are talking about?

One thing I just noticed while looking is that the schematic has you taking B+ from pin 2 of the rectifier tube. For directly-heated rectifier tubes like the 5Y3, it makes no difference, but since the cathode connection on indirectly-heated rectifier tubes is at pin 8, it's better practice to take the B+ from that one.

Where was your virtual center-tap in the circuit? Some folks do it at V1 where the heater string terminates, but I could see that inducing hum if not laid out perfectly. On the other hand, grounding one side of the heaters definitely caused V1 hum problems for me, no matter what I did with the layout, so I would consider a virtual center-tap using a "humdinger" pot, at the pilot light or one of the output tubes.

Hope this helps!
- Scott

[pdf64]

1M is the standard input impedance for stuff intended for guitar etc, so may as well conform to that, unless there's good reason not to.

The 25k HT bleeder is shown at the OT CT; values >100k are normally used.

You may be fine without grid stoppers, but the amp may have a low margin of stability, and perhaps sometimes an innocuous as replacing the power tubes could be enough to introduce some degree of instability.

It's strange that balancing the heaters was noisier than running them unbalanced.

[Rob's Radio-Active]

Thanks. Scott. I'm not a tube-roller, so it never occured to me to take B+ from pin 8. I guess I figure if someone want's a tighter PS from a 5AR4, they can switch pins.
Good point about the pot on the virtual CT, but I didn't have any low value pots except for, like 25 ohm, wirewounds. Anyway, the hum issue is resolved and the owner is quite pleased with the sound, I will ceratianly note both you guys' good ideas on the next amp.

...On the original bass issue: The owner say's the bass quantity and quality on this unit puts the Princetone to shame in side by side tests. I think the extra iron was worth the money. Says it sounds closer to his Blues Deluxe Reissue.

Cheers,
Rob

[pdf64]

I think that when Marshall built Vox amps used the rectifier terminal #2 for the HT output, they tended to kill rectifier tubes, as the rectifier heater also carried the HT current.

[Rob's Radio-Active]

Where's the 25k resistor you guys are talking about?

Follow the output X-fmr center tap back toward the power supply. It's on that line to ground.

I think that when Marshall built Vox amps used the rectifier terminal #2 for the HT output, they tended to kill rectifier tubes, as the rectifier heater also carried the HT current.

That is interesting, thank you, but I am not seeing how it changes current flow in this particular arrangement since there is only one complete path for HT current, and that is "perpendicular" to the length of the filament. The 5VAC transformer winding is floating. Am I missing something?

Cheers,
Rob

I am kind of a visual guy. This is how I see it, and switching pin numbers doesn't seem to me to make any difference to filament power handling: