Bias adjustment, Fender 68 Custom Reissue Princeton Reverb
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- Опубликовано: 17 янв 2019
- How I adjust bias on this amp. Not an instruction or tutorial video. This is a recent production Fender reissue 68 Princeton Reverb, made in Mexico
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Thank you for making this video, it is very helpful
Great video, David. I assume the steps would be the same for the blackface 65 Princeton Reissue? I've never biased a Princeton, only the Deluxe/DeVille's. With their test points its very simple. It looks like there are some test points on the Princeton schematic but they appear to be covered in silicon/goop.
Another question: how easy is it to swap out preamp tubes on this amp? Do you have to remove the rear panel to get to them or can you reach up inside (like, for example, you can do with a Hot Rod Deluxe)?
You can do it without removing the panel but its easier to see what you're doing without it :) If you have access to some different 12ax7's then its worth trying a few of them out in the first section, since I was able to notice some differences. I also run a NOS 5751 in the phase inverter slot.
@@davidmilton9616 Thanks David! I haven't got the '68 yet but I will be getting one very soon. In the photos I've seen, it appears the tubes have some sort of metallic cover on them. I hope they're easy to get off! I am actually thinking of putting a NOS GE 5751 in V1, and a NOS Mullard CV4024/AT7 in V2. I have some of these lying around so will try them. But not after making sure the amp is OK first! I did read some reports of reliability issues 'on arrival' with these amps, although most of those complaints were from a few years back. Hopefully Fender have sorted any QC issues out by now!
Do you know if the 68 Custom Vibrolux Reverb has a bias adjustment pot? I heard that they don't, and I've heard that they do. I am in the process of buying a used one and would like to change out the tubes and speakers to get and earlier break up if needed.
Good question. Ive never looked at a 68CVR. Some of the reissues have a port under the chassis where you can adjust the bias pot without removing the chassis. You might look for that as a clue. I don't know why this stuff is so secretive. Obviously it can be very dangerous, you can be killed working inside the chassis, so maybe its a liability issue. However the internet is loaded with a ton of misinformation about these procedures especially if you're reading "forums". You could take it to a good amp tech. Ask him if he can do it in front of you so you can be "trained" for future adjustments. That's always a good way if the tech knows what hes doing and is willing to do that. Thanks.
@@davidmilton9616 I was going to buy the Eurotubes bias probe for octal tubes and do it myself. They have a good video on their site on how to go about using it. Looks like a decent product. I also have a Peavey Delta Blues 115 amp. I know that one is a fixed bias with no trim pot. You have to change a resistor in that one to change the bias. Think they're set pretty cold at the factory so you can get by with a wider range of tube currents.
By biasing it. Will it make is less warm or is that a whole other thing? I toon out the stock (celestion ten 30) speaker and installed a Jensen C10Q.
Adjusting bias is basically optimizing performance of the amplifier by adjusting the electrical current. Level of bias can impact sound. Best to read and study about it. Thank you
Change c24 to .022. The .1 cap on the bass pot is way too high. Trust me it opens the amp up. Also if it has the r34 18k resistor remove and install a lead in its place. Look it up it helps.
Silas Cousteau thanks, I’ll look into it. I know i definitely need to take my gretsch double annny to get it serviced because i feel the output on the guitar is low.
1: is it hum balance or bias balance or biad adjust pot?
2 : what is best bias voltage/current for fender super reverb 1969 (recently i have mod it to ab763) ?
hello,1. it is bias adjustment pot. 2. check here robrobinette.com/Tube_Bias_Calculator.htm Thank you
@@davidmilton9616 so many thanks
Does turning the pot clockwise make the bias hotter?
The only way to be certain is to have a measuring device to read current while adjusting the potentiometer. Doing so without a measuring device can damage your tubes, transformer, or cause numerous other problems. Thanks for watching.
What do you have the chassis resting on to keep the tubes safe?
Its just on the cabinet, which is not something I recommend. I typically have an adjustable wooden rack I made to safely support the chassis. I made this video in a hurry and looking back at it, I didn't take good precautions.. Thanks
@@davidmilton9616 Great way to melt the tolex, ask me how I know LOL
Is that TP 16 and TP 17 you are checking the bias readings on?
No. I'd have to open the chassis again to look but I believe they were R#'s
@@davidmilton9616 OK, thanks.
@@kyp1969 These looked like (in order checked) D2, D3, D1, D5 according to the schematic/circuit photos I have seen. I believe you can use R20 and R32 in place of D1, D5 with the same result. guitarampblog.files.wordpress.com/2017/08/dsc01426.jpg guitarampblog.files.wordpress.com/2017/01/fender-custom-65-princeton-reverb-schematic-vs-68.pdf
I wonder what the R#s are you’re holding on? Critical info in your “not a tutorial” video.
Thanks for watching, I no longer have the unit and I can't remember what the R#'s were. If you can find a schematic it should help you identify the ones you need. Thanks.
R22 and R32 -- just check the schematic
Lots of hum on these, stock, even with the reverb plugs disconnected from the chassis. Pulling v3 makes it go away. Bad lead dress and likely not the proper grounding scheme in this amp.
Yes I agree, mostly poor lead dress. This can be corrected easily. Using good NOS tubes in V2 and V3 helps too. Thanks
@@davidmilton9616 I spent hours with a client's amp on the bench troubleshooting, and this particular issue looks like it may require a different grounding scheme. More to come... this is apparently a very common issue with these amps.
I do have to say that no movement of any wires that are present in the amp made any affect whatsoever on the hum.
@@ChonkTek I usually replace all the filter caps with F&T's too. That always helps. Thanks for watching and your comments.