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PostPosted: Thu May 14, 2020 2:44 pm 
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In this post, Stephen said:

coco wrote:
If you installed the bias switch correctly per the layout, with the switch to the left (preamp side), the 18V zener gets shorted out for KT66 operation. Bias switch pointing to the right, zener in circuit, is the 6V6 position.

KT66 <<TOGGLE>> 6V6


Strictly from a mechanical perspective, is there a right/wrong physical orientation of the switch? In other words, ignoring the wiring, is there a "right side up" and an "upside down", or does it not matter? If I correctly understand the switch and how it's used here, I don't think it matters.

It looks like there might be an inconsistency between the schematic and the layout diagram. In particular, for the layout diagram, I think the "6V6" and "KT66" labels for the switch are backwards. The "6V6" label is by the switch position that shorts out the zener diode and brings R44/620k into the circuit, but isn't that for KT66 operation?

Also, I want to check my understanding of these switches. I am assuming the part that moves is on a hinge, so that when it's to the "left" the leads on the "right" are connected, and vice-versa. Here's an ascii art picture of what I'm trying to say:

Code:
 Position 1             Position 2
          /                 \     
         /                   \   
        /                     \
+-------------+        +-------------+
|     /       |        |        \    |
|    /        |        |         \   |
|   /         |        |          \  |
+-------------+        +-------------+
  |    |    |            |    |    |
  A    B    C            A    B    C
 
A-B shorted            B-C shorted
C not connected        A not connected


Is that right?

Lastly, quoting from the same post as above:

coco wrote:
Note: If the amp is fitted with KT66s and the switch moved to the 6V6 position, that could damage things, since KT66s require a higher bias voltage. With 6V6s, there is no danger with the switch in either position, although the amp won't sound its best, since the internal voltages will be a little too high.


Can you elaborate on that? I would assume the opposite would be true, that with KT66 tubes installed, if you switch the 6V6 position, you'll drop bias an B+ voltages, which might sound bad, but presumably wouldn't hurt things? I'm having trouble getting my head around it!

Thanks!


Last edited by matt_garman on Fri May 15, 2020 1:05 pm, edited 1 time in total.

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PostPosted: Fri May 15, 2020 12:09 am 
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Electrically, there's no right or wrong way to install the switch. It just depends on which way you want the handle to point for each setting.

The layout drawing is correct. The labels show which way the bias switch handle will point for each setting. The labels are opposite to which terminals are connected though.

Your drawing is correct, too. The handle points opposite to the terminals that get connected. This can be easily verified with an ohmmeter. Just see which way the handle points to connect the center terminal to the outer ones.

Quote:
coco wrote:
Note: If the amp is fitted with KT66s and the switch moved to the 6V6 position, that could damage things, since KT66s require a higher bias voltage. With 6V6s, there is no danger with the switch in either position, although the amp won't sound its best, since the internal voltages will be a little too high.


I'm not sure about this either but I would hate to disagree with Stephen. KT66 gets higher B+ than 6V6. Hotter bias too, as far as I know, but I don't have a Triwatt to verify this. So I would think KT66 wouldn't get damaged on the 6V6 setting. 6V6 might not like the KT66 setting, though.


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PostPosted: Fri May 15, 2020 10:45 am 
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mitch m wrote:
The layout drawing is correct. The labels show which way the bias switch handle will point for each setting. The labels are opposite to which terminals are connected though.


Great, thanks for the clarification. My problem was the statement, "with the switch to the left (preamp side), the 18V zener gets shorted out for KT66 operation", I thought he meant the handle part of the switch, but he meant the internal/terminal part of the switch. But the in layout diagram, the labels are indeed for the handle part of the switch.

In short, you have to be explicit when talking about switch position, i.e. are you referring to the external/handle part of the switch, or the internal/terminal part?


mitch m wrote:
Your drawing is correct, too. The handle points opposite to the terminals that get connected. This can be easily verified with an ohmmeter. Just see which way the handle points to connect the center terminal to the outer ones.


Great! After posting, I did indeed verify this operation with my DMM...

mitch m wrote:
I'm not sure about this either but I would hate to disagree with Stephen. KT66 gets higher B+ than 6V6. Hotter bias too, as far as I know, but I don't have a Triwatt to verify this. So I would think KT66 wouldn't get damaged on the 6V6 setting. 6V6 might not like the KT66 setting, though.


...I think I understand this now, see the latest post in my Triwatt build thread. I'm not 100% confident my explanation is right, so I don't want to repeat potentially bad info here if I'm wrong (damage control for my ego at this point ;)).


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