• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Switch Issues - 2 amplifiers (SS + Vacuum Tube) to 1 pair of speakers

Xulonn

Major Contributor
Forum Donor
Joined
Jun 27, 2018
Messages
1,828
Likes
6,311
Location
Boquete, Chiriqui, Panama
The problem is that vacuum tube amplifiers can be damaged or destroyed if powered on without a load on the speaker terminals. Therefore the switch should not only switch amplifiers, but also switch the unused amplifier to a dummy load to protect the vacuum tube amplifier in case you accidentally leave it on - or turn it on.

[Edit: After more research, I discovered that the danger from no-load on the output transformers amplifiers of a vacuum tube aplifier is probably not an issue for many configurations and situations, at least when the amplifier is an ultralinear tetrode, pentode design utilizing negative feedback (NFB), not operating in triode mode and not driven into clipping. See more on this in my February 16 post below.]

After an extensive internet search, I found many source/amplifier/speaker switches, but none with tube amp protection. However, I did discover a little Canadian company in Ontario that builds vacuum tube amplifiers and custom switches. (An additional benefit is that their design style would complement my black with wood panel YarLand amplifier very nicely.)

Mapletree Audio Design [LINK] offers a unique range of designed and hand crafted by Dr. Lloyd Peppard and Dr. Al Freundorfer, both electrical engineering professors. Our goal is to foster the enjoyment of recorded music through the use of vacuum tube amplification and signal processing, offered at a reasonable price.

I sent them the below email, and invite ASR members and visitors to watch or join my quest.

To: Mapletree Audio Design:

As you know, vacuum tube amplifiers can be damaged or even destroyed if turned on/operated without a load on the speaker outputs.

Therefore I am looking for a two-amplifiers to one pair of speakers switch box with XLR input and output, and 5-way speaker connectors, using high-quality (but not esoteric) switches and connectors. I will use my 100wpc ICEPower SS amplifier for HT/Video use plus casual background listening, and my PP, Class A, EL34/6CA7 YarLand vacuum tube amplifier for dedicated and focused music listening.

When switching amps, I want to protect the tube amplifier in case it is accidentally left on when switching from the speakers to the SS amplifier.

The core of my requirements is to switch the vacuum tube amp to a dummy load (internal or external resistors, or a pair of super cheap tiny 8Ω speakers), thereby protecting the tube amp from damage if it is left on. It would also be nice to have a flashing red LED warning light (by-passable) if there is current from the tube amplifier to the dummy load indicating that the tube amp is powered on when the SS amplifier is being used.

Of course, some tube amp aficionados want to leave their amps on, and a dummy load is mandatory, but unlike Class AB amplifies, Class A vacuum tube - and SS amplifiers - are so energy inefficient and hot-running, that I want to save energy and reduce heat by turning the amplifier off when not using it.


Mapletree Audio Design - Switch.jpg
 
Last edited:

antcollinet

Master Contributor
Joined
Sep 4, 2021
Messages
7,409
Likes
12,294
Location
UK/Cheshire
What would worry me (and bear in mind that I know nothing about tube amps so no idea if this is a problem or not.

The switch needs to first disconnect one amp, and then connect the other - or it risks connecting the output of both amps together. (Does it do this "break before make"?)

Assuming it does, that means your tube amp will still be connected to nothing for the duration between break and make. How long does it take to damage an unloaded tube amp? What about switching while playing music - That is highly likely to cause huge voltage spikes due to the current in the speaker voice coils suddenly having nowhere to go..
 

Blumlein 88

Grand Contributor
Forum Donor
Joined
Feb 23, 2016
Messages
20,524
Likes
37,057
A 220 ohm resistor rated for a few watts permanently across the tube amp output is enough to protect it. Leave in place at all times and you'll never know it is there otherwise. Then any regular speaker switch breaking before making should work.

If your amp has multiple impedance taps it is marginally better to put it on the 16 ohm or whichever tap is highest.
 
Last edited:
OP
Xulonn

Xulonn

Major Contributor
Forum Donor
Joined
Jun 27, 2018
Messages
1,828
Likes
6,311
Location
Boquete, Chiriqui, Panama
A 220 ohm resistor permanently across the tube amp output is enough to protect it. Leave in place at all times and you'll never know it is there otherwise. Then any regular speaker switch breaking before making should work.
@Blumlein 88 - thanks - Such a simple answer. I think you just saved me a lot of money. What wattage resistor? I could solder them myself inside the switch box.
 

Blumlein 88

Grand Contributor
Forum Donor
Joined
Feb 23, 2016
Messages
20,524
Likes
37,057
@Blumlein 88 - thanks - Such a simple answer. I think you just saved me a lot of money. What wattage resistor? I could solder them myself inside the switch box.
5 watts should do it. 10 watts would be plenty. I think I've used 2 watt in the past, but giving someone else advice a little extra is good insurance and no big increase in cost.

As to putting them in the switchbox I don't know what room you have there. I'd just hang them off the back of the amp speaker connectors. That way you aren't dependent upon any switching. It is there all the time and won't bother anything when the speakers are connected.

BTW, here is an excellent highly detailed article on protecting output transformers and tube amps from various fault conditions. Almost no (to my knowledge none) of the modern tube amps employ these techniques or at most only one of them. Way down in the middle of the article they mention loading the output with a higher value resistor to protect the OPT.

 
Last edited:
OP
Xulonn

Xulonn

Major Contributor
Forum Donor
Joined
Jun 27, 2018
Messages
1,828
Likes
6,311
Location
Boquete, Chiriqui, Panama
5 watts should do it.

As to putting them in the switchbox I don't know what room you have there. I'd just hang them off the back of the amp speaker connectors. That way you aren't dependent upon any switching. It is there all the time and won't bother anything when the speakers are connected.
Solution A, then, is your suggestion for resistors, plus these two ($65+$68 at Amazon) switch boxes, with the amplifier switch tested and recommended by @amirm - unless I can find a single-box solution. (Interesting that the Goldpoint 1<->2 XLR switch box costs $433!)

1-In + 2-Our Amplifier Switch.jpg

========================================================================

2-In + 1-Out Speaker Switch.jpg
 
OP
Xulonn

Xulonn

Major Contributor
Forum Donor
Joined
Jun 27, 2018
Messages
1,828
Likes
6,311
Location
Boquete, Chiriqui, Panama
I just updated my YarLand/Ariand thread and mentioned that I will still be switching my speakers back and forth from a Class-D ICEPower SS amplifier to a Class-A EL34 Tube amplifier, but will use separate sources, and I will not need a source switch.

As I mentioned in my edit to the original post in this thread, more online research lead me to believe that the danger from no-load on the output transformers amplifiers of a vacuum tube amplifier is probably not an issue for many configurations and situation. At least when the amplifier is an ultra-linear tetrode, pentode design that uses negative feedback (NFB), is not operating in triode mode and is not being driven into clipping. There are still lots of caveats, and I can see why, because of the potential for misuse by customers and subsequent legal action or reputation bashing, many small audio companies won't sell speaker or speaker plus source switches. And the market is too small for the big players in audio.

Depending on what responses I get from our resident ASR experts, I will follow @Blumlein 88 's advice, and just for a little margin of safety, put a "safety resistor" across the speaker terminals, especially since my amplifier is a switchable dual mode model - triode or pentode.

Here's a LINK to the best discussion of this that I found, and below is a summary of their conclusions.

Rules of Operating Vacuum Tube Amplifiers Without a Load

1. These rules only allow amps with negative feedback (NFB) to be operated without a load. Guitar amps and hifi amps which have little or no NFB are not considered here. The reason for this is that the NFB constrains the amp to a fixed value of gain. The gain of non-NFB amps may rise greatly under no-load conditions.

2. Only amps which are stable without a load can be operated that way. Most NFB amps are less stable without a load. Some might actually oscillate. It would be very rare for a properly working commercial amplifier to do that, though.

3. All amps which are not driven into clipping have no risk of damage. As long as a pentode or ultralinear amp isn’t driven into clipping, its screen grids will not draw excessive current. Similarly, amps not driven into severe clipping will not suffer inductive spikes which are otherwise possible.

4. Ultralinear amps should not be driven into clipping in unloaded conditions, because the screen swings typically only 43% of the plate. That leaves the screen voltage at a high enough value to cause damage, when the plate bottoms-out.

5. Pentode amps do risk damage, when driven into clipping. Screen dissipation could rise high enough to do damage, in continuous operation while clipping. This is because the plate voltage pulls low and the screen remains steady, diverting a large amount of current to the screen.

6. I found in the lab that unloaded triode amps driven into severe clipping by lower frequency square waves do risk damage from inductive spikes. There may be other such cases involving clipping.

The bottom line: You can operate safely, any stable, NFB, vacuum tube amp without a load, as long as you don’t drive it into clipping.

One useful result: There is no need to connect a load to a normal vacuum tube amp if you just want to adjust bias.
 

Blumlein 88

Grand Contributor
Forum Donor
Joined
Feb 23, 2016
Messages
20,524
Likes
37,057
I just updated my YarLand/Ariand thread and mentioned that I will still be switching my speakers back and forth from a Class-D ICEPower SS amplifier to a Class-A EL34 Tube amplifier, but will use separate sources, and I will not need a source switch.

As I mentioned in my edit to the original post in this thread, more online research lead me to believe that the danger from no-load on the output transformers amplifiers of a vacuum tube amplifier is probably not an issue for many configurations and situation. At least when the amplifier is an ultra-linear tetrode, pentode design that uses negative feedback (NFB), is not operating in triode mode and is not being driven into clipping. There are still lots of caveats, and I can see why, because of the potential for misuse by customers and subsequent legal action or reputation bashing, many small audio companies won't sell speaker or speaker plus source switches. And the market is too small for the big players in audio.

Depending on what responses I get from our resident ASR experts, I will follow @Blumlein 88 's advice, and just for a little margin of safety, put a "safety resistor" across the speaker terminals, especially since my amplifier is a switchable dual mode model - triode or pentode.

Here's a LINK to the best discussion of this that I found, and below is a summary of their conclusions.

I'd not try it with nothing on the output. If you get a bad reaction it arcs or shorts windings. It may not do enough to immediately stop the amp from working, but if it happens enough bad things happen among them destroying your transformer. With ultralinears and pentode connections you can probably even get by fine with 470 ohms across the outputs instead of the 220 ohm 5 watt resistor I suggest. If you plan on switching loads on and off your tube amps its is just too easy and cheap to protect the most important part that being the output transformers. Put the resistor across the highest impedance tap if you have multiple taps.

@SIY is someone who can give you an expert opinion.
 
Last edited:

LTig

Master Contributor
Forum Donor
Joined
Feb 27, 2019
Messages
5,760
Likes
9,442
Location
Europe
I'd not try it with nothing on the output. If you get a bad reaction it arcs or shorts windings. It may not do enough to immediately stop the amp from working, but if it happens enough bad things happen among them destroying your transformer. With ultralinears and pentode connections you can probably even get by fine with 470 ohms across the outputs instead of the 220 ohm 5 watt resistor I suggest. If you plan on switching loads on and off your tube amps its is just too easy and cheap to protect the most important part that being the output transformers. Put the resistor across the highest impedance tap if you have multiple taps.
I absolutely agre with you. I do wonder though why those amps don't come with such bleed resistors installed inside. I would do it if I would sell them.
 
OP
Xulonn

Xulonn

Major Contributor
Forum Donor
Joined
Jun 27, 2018
Messages
1,828
Likes
6,311
Location
Boquete, Chiriqui, Panama
I absolutely agree with you. I do wonder though why those amps don't come with such bleed resistors installed inside. I would do it if I would sell them.

That's exactly what puzzles me. It's easy for speaker wires - bare wire, or terminated with spade lugs or banana plugs - to fall out or get knocked or pulled out by accident. So why not include cheap protection? The bleed-off power drain is minimal. It seems logical to me that bleed resistors should have been included as a safety standard for tube amps starting back in the 1940's or 50's.

Using a speaker switcher this far in time after the heyday of vacuum tube amplifiers should not require as much effort as I had to put into finding out the facts and determining safety procedures.

I just added a $7 bag of 15 resistors to my monthly Amazon order.
41nwR9bVPBL._AC_SL1000_.jpg
 

SIY

Grand Contributor
Technical Expert
Joined
Apr 6, 2018
Messages
10,386
Likes
24,749
Location
Alfred, NY
If you’re not feeding signal to the input, having no load will not cause an issue for an amp that is stable. The dummy resistance is often even built in.

If your switchbox doesn’t switch inputs, get a different switchbox.
 

petek

New Member
Joined
Mar 16, 2022
Messages
1
Likes
0
You may find you dont get a response from MapleTree or a response that states "not willing".

I wanted them to build me something similar to the Niles 3 tape deck switcher. I contacted them a few weeks ago.
Finally I just did get a response saying--why not repair it? My business partner (Lloyd) is going blind, taking care of him takes all my time. It would take me 3 months to even get to it...

...maybe your result will vary.
 
OP
Xulonn

Xulonn

Major Contributor
Forum Donor
Joined
Jun 27, 2018
Messages
1,828
Likes
6,311
Location
Boquete, Chiriqui, Panama
If your switchbox doesn’t switch inputs, get a different switchbox.
Different/separate sources - an Intel NUC HTPC (LibreElec/Kodi) connects to an IOTAVX AVP which drives the ICEPower amp, and a RPi with MoOde > Topping DX7s DAC feeds the YarLand tube amp. Only the speakers will be switched.

I'm comfortable with the resistors solution.
 
Top Bottom