Princeton Reverb Build Thread

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Re: Princeton Reverb Build RAW control help?

Post by ziess » Mon Oct 31, 2016 9:55 am

Cool, glad it helped. Let me know how you get on.

Tommy.

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Re: Princeton Reverb Build RAW control help?

Post by salty » Mon Oct 31, 2016 6:39 pm

So I am moving along the back of the board attaching the leads now. As promised a picture of progress.

Image


I have stopped for a bit to do some research. The transformer set I got for the build are the Classic Tones from Magnetic Components and it has an 8 and 4 ohm tap for the OT.
I had a Hot Rod deluxe that I had to repair the external speaker out jack on and I saw the cool trick they used for the multi-tap OT on that amp.
I think I want to use that trick for this build. Basically the two output jacks were wired so that when nothing was plugged into the external jack, the internal jack saw the 8 ohm tap and when something did get plugged into the external jack, it broke the connection to the 8 ohm tap and gave the jacks the 4 ohm tap.

I have an extra 1x12 cab with an 8 ohm speaker in it, so wiring it this way would let me run both that 1 x 12 and the internal speaker on the 4 ohm tap and give the amp a little more cone to drive.

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Re: Princeton Reverb Build Thread

Post by ziess » Tue Nov 01, 2016 3:24 pm

Looking good! I'd need to look at the schematic again, but my recollection is that the HRDlx jacks are either 8Ω internal speaker jack or 4Ω external with a dummy plug in the internal jack. Honestly, it's a drama waiting to happen. I prefer to go for fool proof options. Why not just use a switch?

Decent transformers won't really care about a 100% mismatch anyway, and 8Ω down to 4Ω is about as safe as it gets (I've run my '62 brown Deluxe that way for years, for instance). Remember that impedance is a curve that varies by, amongst other things, frequency. Any good quality OT will comfortably tolerate that load (though you may still prefer the other winding, tonally speaking, though I'd suspect that's less likely given you'll only be using half the secondary winding), as long as you're not driving the amp into to the floor for 8 hours a day. Do you know the turns ratio (plate load:speaker load) of the OT? I'd bet it's rated for 8K (possibly 6K6) : 8/4Ω. Assuming you've got c.300VDC on your output tube plates then a pair of 6V6GTs will want to see a load of approx 6K6Ω (so a turns ratio of 1:825 for an 8Ω load). Dropping that to 8Ω will present a c.4K load, something I'd consider within the limits of a decent set of tubes (see page 5 here http://www.tubebooks.org/tubedata/HB-3/ ... V6-GTA.PDF" onclick="window.open(this.href);return false;), and a decent OT. You'll note from page 6 on the GE spec sheet that power output is reasonably steady between about 5K5Ω - 8KΩ plate load - http://www.radiostation.ru/tubes/6V6GT-GE.pdf" onclick="window.open(this.href);return false;.

Basically, it'll probably be ok with good quality (old!) matched tubes, it'll just run a wee bit hotter.

Tommy.

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Re: Princeton Reverb Build Thread

Post by salty » Thu Nov 03, 2016 7:55 am

Thanks so much for all the information!

So the way the Hot Rod Deluxe is that it has two different types of jacks for the outputs.

The external one is a: JACK, 1/4", TIP-SLEEVE, WITH TRANSFER SWITCHING. Switchcraft 13A Jack.
And the internal is a: JACK, 1/4", MONO, CLOSED CIRCUIT (SHORTING), WIRED, Switchcraft® 12A Jack.

So when nothing is plugged into the external jack is bypasses the 4Ohm tap and the internal jack uses the 8 Ohm tap.
When you plug something into the external jack, both jacks become active and the 4 Ohm tap is used.

My understanding of it is that yeah, I could just wire up the 8 Ohm tap and be okay running both an internal and external speaker for a 4 Ohm load and like you said not run it wide open for long periods of time.
But I thought that if you do that, it actually reduces the power the speakers get by like 25%. Is that correct?

With the way I am doing it with the Hot Rod output jack wiring, the OT will sue the 8 Ohm for just the internal speaker and a 4 Ohm tap when both are connected.

Is that a bad idea?

thanks so much for all your help! Building amps is fun, but I am having to do a lot of reading as I go. :)

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Re: Princeton Reverb Build Thread

Post by salty » Thu Nov 03, 2016 10:36 am

More progress. Had found a mistake and fixed it, somehow I missed the wire coming from the .02 uF cap to pin 1 of the PI. Eyelets make it so easy to fix that stuff!

Not quite as clean as I was hoping for based on other pics I have seen people do, but I think it still looks better than some of the factory Fender ones I have seen the inside of.

Image

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Re: Princeton Reverb Build Thread

Post by salty » Fri Nov 04, 2016 10:58 am

I went ahead and wired it up using the same jacks and wiring as the hotrod deluxe uses.

Image

When nothing is plugged into the left jack, it shoots the 8 Ohm tap to the right jack. When something gets plugged into the left jack it shares the 4 Ohm tap between the two jacks.

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Re: Princeton Reverb Build Thread

Post by salty » Sat Nov 19, 2016 12:26 pm

Okay, got all the speaker stuff sorted and working, but something is really wrong with my bias circuit. :(

I noticed that I am not getting enough negative bias voltage.... -3 instead of the expected -30... as a result the plate voltage on my power tubes is low. From what I know everything looks right in the bias circuit itself, but I see something odd related to my PT.

I am using the Magnetic Components, Inc 40-18019 Power transformer. The specs are here: http://www.classictone.net/40-18019.pdf

That document shows with a incoming voltage from the wall of 120 Volts, I should see 650 Volts across the two Red wires that provide voltage to the rectifier and bias circuit. Instead, I am seeing 630 volts. Does that mean my transformer is not correct? Or is there something I do not understand that plays into the transformer putting out the expected 650 volts?

Would that being low by 20 volts not provide me the negative bias voltage I am expecting?

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Re: Princeton Reverb Build Thread

Post by ziess » Mon Dec 19, 2016 5:36 am

Verify the value of the 100K resistor on the bias board and check the diode is the right way round. What's your bias voltage at the output of the bias board?
Definitely 120VAC line voltage?

Tommy.

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Re: Princeton Reverb Build Thread

Post by Racing » Mon Dec 19, 2016 6:50 am

1/ Do not run the amp with powertubes installed until the situation is cleared.

2/ Check bias circuit for DC resistence vs ground. Do so ahead of its diode and do so at its DC points. The AC one should show the resistence of the winding vs the centertap of the powertransformer. Should the DC side show real low values vs ground start by suspecting the tremolo pot and the electrolyte involved. Need be desolder either and recheck.

3/ As per above checks out, yank powertubes. Switch amp on. Check to see that there´s "raw" AC voltage inbound for the bias circuit. If not,find out why.
Check DC voltage at the electrolyte. If you want to make bias adjustable replace the 22k resistor with a 50k trimpot of 1/4 watt. In such a scenario install two pcs of 1 ohm resistors of 1w between each powertube socket pin 8 and chassis ground. This provides a good measuring point.

Ohms two laws.
U=R*I
Where U= volts. R= resistence in Ohms and I= amperage in amperes.
P=U*I
Where P= watts.

As the "bias resistors" (the ones installed by you between pin 8 and ground) are of 1 Ohm we can directly "translate" the voltage drop across them from mV to mA. Put in context... U=1*I and something times one...you get the drift. By measuring this we in short conclude the amount of current across that tube in idle.
Now we measure B+ at the output transformer centertap and multiply that with our reading above.
U*I=P
This way we´ll know the powerloss of that tube in idle. The 50k pot vs ground within the bias circuit that we replaced the stock 22k resistor with lets us dial the actual negative bias voltage to the potential needed to make them powertubes come to rest where WE want them to.
When altering a bias circuit like this ALWAYS replace the resistor vs ground. Reason is simple,should that trimpot F up somewhere down the line (read-break for whatever reason) all that happens is that the powertubes will see an increased negative bias voltage,net result being that worst case scenario this voltage will become so high that the powertubes will go into what´s known as cutoff.
IOW..just replace the trimpot and go at it again. No harm no foul in short.

Adjust each powertube to an anode loss in idle of approx 9 watts and you´ll be home free.

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Re: Princeton Reverb Build Thread

Post by salty » Mon Dec 19, 2016 6:55 am

Hey guys,

Sorry about this. I actually resolved the problem and forgot to update my post. I had the bias diode in backwards. On the layout, I looked at the + and - symbols instead of focusing on the diode symbols direction. :( Ruined a good set of tubes with it, but learned a 50 dollar lesson. :)

I have had the amp up and running for about 2 weeks now and it is working perfectly and sounds great.

I do wish I would have put a longer power cord on it and will do that when it is time to replace the tubes.

I will add some pictures soon as well.

Racing, thanks for the reply.

Tommy, thanks for all the help from the start!

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Re: Princeton Reverb Build Thread

Post by ziess » Mon Dec 19, 2016 9:16 am

No worries, I knew it would be something simple. How is the raw control?

Tommy.

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Re: Princeton Reverb Build Thread

Post by salty » Mon Dec 19, 2016 9:30 am

Haha, yeah silly mistake on my part.

So far I prefer the stock setting or just a tad of the raw control used. It causes the amp to break up when turned up much and it really is not a super pleasant sound to my ears.

I have read there are things to get the break up sounding better, but I think I am happy with the clean sound and pedals.

I forgot how great the Princeton takes pedals compared to my AC30. The AC30 can sound okay wit several drives, but it jsut does not impart the amp in the box character pedals nearly as well as the Princeton.

I was thinking a 5e3 would be another nice amp to add to my collection, but now I am wondering about something like a single channel super reverb in a 1x12.

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Re: Princeton Reverb Build Thread

Post by jthomas » Mon Dec 19, 2016 9:47 am

Yep. It's clear... you're addicted (to building amps).

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Re: Princeton Reverb Build Thread

Post by salty » Mon Dec 19, 2016 10:06 am

jthomas wrote:Yep. It's clear... you're addicted (to building amps).
Lol. I think I may be, it seems to feel more rewarding then building a effects pedal in a lot of ways. It is not nearly as affordable though. :)

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