Carverfest Tube Amp Technical Discussion

12th Annual CARVERFEST 2018 September 5th - 19th
User avatar
UncleMeat
TFM-25
Posts: 328
Joined: Thu May 21, 2015 5:32 pm

Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Sat Sep 22, 2018 4:17 pm

RobertR's instructions for assembly, re-posted here for reference.
My CB instructions.zip
CF 275 Tube Amp Assembly Instructions
(106.55 KiB) Downloaded 27 times

Morris
Newbie
Posts: 21
Joined: Sun Feb 25, 2007 4:58 pm
Location: Oklahoma

Re: Carverfest Tube Amp Technical Discussion

Post by Morris » Sun Sep 23, 2018 4:09 am

Here are some pics of the amp SteveK built at the fest. My pics are all of the bottom. SteveK has some of the top.
https://www.flickr.com/photos/160853170 ... 8251925912

packratt
M-400t CUBE
Posts: 176
Joined: Sat Jun 25, 2005 10:00 pm

Re: Carverfest Tube Amp Technical Discussion

Post by packratt » Sun Sep 23, 2018 8:53 am

Any suggestions for cheaper tubes to do the initial start up with?

Would sucked to fry the kt100 due to a wiring screw up

Morris
Newbie
Posts: 21
Joined: Sun Feb 25, 2007 4:58 pm
Location: Oklahoma

Re: Carverfest Tube Amp Technical Discussion

Post by Morris » Sun Sep 23, 2018 10:06 am


Morris
Newbie
Posts: 21
Joined: Sun Feb 25, 2007 4:58 pm
Location: Oklahoma

Re: Carverfest Tube Amp Technical Discussion

Post by Morris » Sun Sep 23, 2018 10:11 am

I need instructions on biasing , both the KT120's and other tubes like the cheap EL34's.

packratt
M-400t CUBE
Posts: 176
Joined: Sat Jun 25, 2005 10:00 pm

Re: Carverfest Tube Amp Technical Discussion

Post by packratt » Sun Sep 23, 2018 11:26 am

[quote=Morris post_id=215967 time=1537715172 user_id=2996]
Cheap disposable tubes...
https://www.ebay.com/itm/NEW-Shuguang-E ... 3159!US!-1
[/quote]

Thanks.
How about for the 12ax's?

This is my first foray into tubes, so I don't really know what's going on.

Morris
Newbie
Posts: 21
Joined: Sun Feb 25, 2007 4:58 pm
Location: Oklahoma

Re: Carverfest Tube Amp Technical Discussion

Post by Morris » Sun Sep 23, 2018 11:55 am

I plan to use the 12AX7 12AT7's that came with the kit. I have some vintage tubes to try after final testing.

User avatar
UncleMeat
TFM-25
Posts: 328
Joined: Thu May 21, 2015 5:32 pm

Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Sun Sep 23, 2018 1:03 pm

The JJ 12AX7 and 12AT7 are not expensive tubes. They shouldn't be damaged if you followed the installation of resistors correctly.

Please be careful when probing or working inside of these amps, the B+ voltage is up around 700VDC. Make sure the amp is unplugged/drained before making any changes (use a DMM to check that bias supply caps are drained first).

User avatar
UncleMeat
TFM-25
Posts: 328
Joined: Thu May 21, 2015 5:32 pm

Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Sun Sep 23, 2018 1:07 pm

Morris wrote:
Sun Sep 23, 2018 10:11 am
I need instructions on biasing , both the KT120's and other tubes like the cheap EL34's.
When you turn the amp on, wait 30sec-1minute and watch the mA meter on the front. It should rise up slowly and then stop rising between 80mA and 120mA. Set the bias to 80-100mA (for KT-120's) using the bias pot on the back of the amp with a flathead screwdriver.
Last edited by UncleMeat on Sun Sep 23, 2018 7:05 pm, edited 1 time in total.

User avatar
UncleMeat
TFM-25
Posts: 328
Joined: Thu May 21, 2015 5:32 pm

Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Sun Sep 23, 2018 7:04 pm

BarryG wrote:
Wed Sep 05, 2018 9:04 pm
More from RobertR:

BTW, there is one error in the PS schematic - the B- voltage should read 47 volts, not -41.
That B- voltage should be NEGATIVE. If it's -47 volts DC you're good. If you have anything + there (or AC voltage) the tubes will overheat.

User avatar
BarryG
Newbie 50+
Posts: 97
Joined: Thu Jul 24, 2014 4:00 am

Re: Carverfest Tube Amp Technical Discussion

Post by BarryG » Sun Sep 23, 2018 9:12 pm

UncleMeat wrote:
Sun Sep 23, 2018 7:04 pm
BarryG wrote:
Wed Sep 05, 2018 9:04 pm
More from RobertR:

BTW, there is one error in the PS schematic - the B- voltage should read 47 volts, not -41.
That B- voltage should be NEGATIVE. If it's -47 volts DC you're good. If you have anything + there (or AC voltage) the tubes will overheat.
I think Robert could have been more clear, I read it as B- meant negative with a magnitude of 47v!

Good catch UM!

BarryG
Carver C-11 (BillD modified by Dennis Miller), Carver SD/A-390t, Carver C-9, Carver M-1.0t MKII Opt. 2, Carver M-500t MKII (RichP’s final MKII before retirement), Carver TFM-45, TEAC DAC, Tube Buffer (MusicFidelity Clone), Sony Blu-Ray Player, Technics SL-7 turntable with Technics 310MC moving coil cartridge, Polk SDA1B’s (upgraded and modded by me), Carver ALS (Original upgraded), Interconnects by Doug, Speaker Wires by Doug, Salamander Archetype Equipment rack X2.

User avatar
UncleMeat
TFM-25
Posts: 328
Joined: Thu May 21, 2015 5:32 pm

Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Mon Sep 24, 2018 12:34 am

We need to be specific on this one Barry. The bias voltage on a tube amp must always be negative in value (usually -30V to -50V). It's the "throttle" on controlling the electron flow in the tubes. The more negative the voltage, the lower the electron flow across the tube. 0V or anything + would equate to an uncontrolled flow of electrons.

Morris
Newbie
Posts: 21
Joined: Sun Feb 25, 2007 4:58 pm
Location: Oklahoma

Re: Carverfest Tube Amp Technical Discussion

Post by Morris » Mon Sep 24, 2018 5:57 am

More cheap disposable tubes. SHUGUANG 6L6GC. I have no idea how to bias them.
https://www.ebay.com/itm/NEW-SHUGUANG-6 ... 3159!US!-1

User avatar
UncleMeat
TFM-25
Posts: 328
Joined: Thu May 21, 2015 5:32 pm

Re: Carverfest Tube Amp Technical Discussion

Post by UncleMeat » Mon Sep 24, 2018 10:43 am

The idle bias is a low current setting to keep the tubes warm but not dissipating much energy. Max dissipation of a 6L6GC is about 35% of a KT120, so you would probably want the idle bias down around 40-50mA. That would require a larger resistor at R58. Otherwise just for testing purposes you can probably just leave the existing idle bias setting at 80mA

Morris
Newbie
Posts: 21
Joined: Sun Feb 25, 2007 4:58 pm
Location: Oklahoma

Re: Carverfest Tube Amp Technical Discussion

Post by Morris » Mon Sep 24, 2018 11:31 am

Thanks Russ. I think it would be a good idea to make sure R58 doesn't have any leads bent over before the board is mounted. It may have to be replaced after assembly.

Post Reply

Return to “CARVERFEST 2018”