Mod'ing a tube circuit (from early 60's stereo turntable)

[TubeNoob]

Ok, seems to be that the thing on the top and side of the PT that the black AC is connected to is a 'spring-loaded rose metal fuse'.

Couldn't find any info on them in a few initial searches. No biggy, if it ain't broke i won't fix it.

So, as for where to attach the ground of the new power cable, since this is a wood cab the only really obvious grounding spot i'm seeing available is where the speaker jacks are. Is this a good spot?

[TubeNoob]

I seem to remember grounding my PS stuff to the cathode ground of my 6V6 on my (single-ended) 5F2A Fender Princeton build.

Found a different service manual for this model, by the way:
http://www.kevinchant.com/uploads/7/...231/ag9115.pdf

[TubeNoob]

Wired in the new power cable with plenty of slack on the ground wire to go wherever it needs to. Suggestions?

Put in the 10k 1 watt in the power supply instead of the 18k, per Chuck-H!

Most of the time went to trying to screw the PT back into the cab; tight fit!

[TubeNoob]

Hmm, so the 80ma fuse blew.

I have only changed out the power cable and the 10k resistor in the PS per Chuck.

Pilot light and tube heaters are still powering up. No sound/report on the speakers at all.

It's a SS rectifier so maybe its the PS caps?

Will pull tubes and power up with new fuse to see if its the power tubes.

As always, open to input...

[TubeNoob]

The fuse that was in there was rated for .5amps, not 80ma. Oye. It was vaporized inside and the end cap came off when i pulled it out.

[TubeNoob]

Changed out the fuses - back in business!!! I guess they were just too old.

@ChuckH: Already a giant difference just from the change to the input section!
I A/B'd the unaltered channel with it at full volume; no contest - clearly louder with much more body.

(I used a 39k instead of 33k since that's what i had on hand).

I am noticing a bit more life to both channels since changing that PS resistor, too.

Yahooo!!!

[TubeNoob]

Ok, emergency post!

I'm poised with soldering iron in hand, but....

On the schemo C8 should be "330mmf" (330pf).

The actual cap that i just pulled out is 3300pf.

Should i still go with the 500pf cap here or keep the old one or go for another value?

Wouldn't i want to go for a value higher than 3300pf to add more 'upper-mids'?

[TubeNoob]

Here is a little recording of the amp so you can hear where things are at.

The first 20 seconds is the unmodified channel. From about 22 seconds on is the channel with the input mod done.

(you can hear (and see from their waveform) that the mod'd channel has more articulation and starts to break up sooner yet with less compression)

Anyway, regarding my previous emergency post, check out the 2nd one to get an idea of the 'brightness' of the amp at this stage, this link should take you straight to this 2nd take:

https://soundcloud.com/user-34254870...nputmod#t=0:21

[Chuck H]

The treble circuit cap should be 500pf. Be sure you are targeting the right cap as per the schematic. There are a couple of 3300p caps in the stock circuit that are NOT the treble cap. It's possible a 3300p cap was mistakenly installed in the treble circuit too though. But it would definitely have been a mistake in the original construction if that's the case.

Too early to listen to the clips in my home

[TubeNoob]

Ok, that's what i get for slingin' solder around late at night on xmas eve... I put that cap back, replaced the 'right' one this time!

Also did the following:

Changed out C9 (cut cap); Changed V1 cathode resistor; put V1 bypass cap on a switch; put neg feedback on a switch with a 2k instead of 1.5k.

Here's a one minute recording i just did which demos each of the stages sequentially.

1) Un-modded channel
2) Modded channel with V1 bias resistor in play, the 500pf treble cap and the new cut cap to ground (what a HUGE difference already!)
3) Now with the V1 bypass cap engaged
4) Now with the negative feedback resistor engaged

Its sounding pretty killer!

https://soundcloud.com/user-342548702/demo2-2nd-wave

yeah, i know, i'm probably the only guy on here today...

happy holidays everybody!!!!

[TubeNoob]

Happy New Year to everyone!!!

So today I finished all the mods (to one channel) except for the 500pf cap on the volume pot. Changes include:

-Escherton's idea to put a pot in the tone-stack to drive the mids
-Chuck-H's volume pot mod and also changing that cap that was attenuating LF's to the power stage.

Holy cow this thing is screamin'!
WAY more volume now.
The mid-control pot adds a ton of flexibility. (Used a 500k log)

Between the mid-control and the various switches for the mods it is doing a great job of being able to sweep from clean and punchy to dirty and growling.
Exactly what I was after! Thank you guys for the incredible help and suggestions on this!!!! Couldn't have done it without you and I learned a helluva lot in the process.

Remaining stuff: doing the same mods to the other channel, cutting the aluminum face plate, soldering up the power supply for the effects circuits and the circuits themselves. Still a bit to go on this little beast but it'll be a cool secret weapon!

[TubeNoob]

Ok, all the tube circuit mods are done to both channels. Yahoooooooo


One last request for info on a Mod!!!

In order to use the tube amp as an 'effects' device without the speakers, what are the options? 8ohm 5watt resisitor? cathode follower? tapping it before the power tubes?

[Chuck H]

If you like the way the 'whole amp' sounds then you'll want to use the whole amp. Since it's stereo you'll need to disable one side. You can put it on the same switch that engages a split resistive load to the other side. That is, a fat DPDT toggle wired so one pole switches the output of one channel from the speaker out to the resistive load and "recording" jack and the other pole does something like lift the cathodes of the unused channel. The split resistive load should probably be a higher impedance than the 8 ohm specified load because an actual speakers "average" impedance would be higher than that. At least that's become the common MO for resistive attenuators. So maybe a total of fifteen ohms split to send the required -10dB and/or +4dB signal to any effects depending on your requirements. You could use a 15R rheostat too and just adjust it until the effects unit/s are happy and the noise floor is optimized. Speaker emulation would be more complicated and I personally haven't found much happiness in my attempts, but there are supposedly some good products that can be put in the loop for that too. If the circuit sounds too flat and brash (like you're not using a speaker ) something that might help would be to use a series cap and a rolloff cap from the CT of the rheostat or the divider circuits. Something like starting to roll off LF around 100Hz and HF after about 6kHz. If you had, say, a 10R series 5R load divider the series cap would be 100uf and the shunt cap would be 4.7uf. That would seem 'nominal'?

[TubeNoob]

Chuck, you are a true beast of the tube wilderness - thank you very much for that explanation. I dug in a bit to know more and i think it's best that i just build a power-soak as its own device later on!

So, with humble heart, i must confess that my 2nd channel (which i just mod'd to be like the first channel) is not working. I didn't check my changes as I went along by turning on the amp after each mod - i will NEVER do that again

Troubleshooting time!

- I have done continuity checks on all the mods and they seem to be connecting ok. (Wouldn't call this conclusive, though).
- Swapped tubes from 1st to 2nd channel, no difference.
- Checked voltages and compared them to the working channel - they all seem to be similar.
- When checking voltages, I'm getting a report on the speakers from C12, so problem comes from before that i presume.
- V1 tube socket a little dirty, but once i got it into place, voltages were correct yet no sound.
- Getting normal voltage on the V1a plate.
- Input section seems to be ok (input making it to the grid of V1a in continuity test)
- Not getting much of a report back when i touch C8 with the test leads as i do on the 1st channel.

So, i'm guessing the prob is between the input and the volume pot (R2).

As sucky as this is, i'm really hoping to learn some troubleshooting techniques from it, so please feel free to throw me suggestions!!!

[TubeNoob]

I have the most wonderful and embarrassing news all in one fell swoop! I wired the volume pot wrong - i thought it was cranked the whole time!

Everything seems to be working good on both channels!!!

I will try comparing the two soon and report back.

Meanwhile, the dual 15v PS for the FX units is now wired up...