Personal power amps

Guitar Tricks Forum > Tech Talk > Personal power amps

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Joined: 01/18/01

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Rather than waste expensive server space by copying everything over here, I ask that people read through this thread...

http://www.guitartricks.com/forum/showthread.php?threadid=2086

...and then come back here to pick up where we left off. I strongly feel that this is the way of the future.
Lordathestrings
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#1

Rather than waste expensive server space by copying everything over here, I ask that people read through this thread...

http://www.guitartricks.com/forum/showthread.php?threadid=2086

...and then come back here to pick up where we left off. I strongly feel that this is the way of the future.
Lordathestrings
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Take a look at this review of three new Fender guitar amps. Note the remarks about how loud these amps are:
http://www.guitartrader.com/guitar1v1/news/story/010905fenderamps.icl
Yeah, I'm preaching... I'm trying to kick up some dust to get people talking on this forum!
Lordathestrings
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#2

Take a look at this review of three new Fender guitar amps. Note the remarks about how loud these amps are:
http://www.guitartrader.com/guitar1v1/news/story/010905fenderamps.icl
Yeah, I'm preaching... I'm trying to kick up some dust to get people talking on this forum!
Lordathestrings
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Well, I will say it before, I am right behind you on this one. i agree that it might be a bit hard to be able to get a full warm tone with teh sound down to very low levels, but still. It is interesting the way it is written as a good thing that the amps are so loud, I guess it is that sort of "bang for your buck" belief that people have. By the way, would it be possible to put a dial that takes the wattage down on a normal amp? I mean, do the amps have to be meade completely differently? I can't really understand a lot of the techier aspects of that amptone article, so i owuld be interested to know just hoow much adjustment it would take for major companies like Marshall and Fender to start adding this feature on their range of amps.
"Dozens of people spontaneously combust each year, it's just not that widely reported".

#3

Well, I will say it before, I am right behind you on this one. i agree that it might be a bit hard to be able to get a full warm tone with teh sound down to very low levels, but still. It is interesting the way it is written as a good thing that the amps are so loud, I guess it is that sort of "bang for your buck" belief that people have. By the way, would it be possible to put a dial that takes the wattage down on a normal amp? I mean, do the amps have to be meade completely differently? I can't really understand a lot of the techier aspects of that amptone article, so i owuld be interested to know just hoow much adjustment it would take for major companies like Marshall and Fender to start adding this feature on their range of amps.
"Dozens of people spontaneously combust each year, it's just not that widely reported".

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Joined: 01/18/01

Posts: 6242

The voltage from the power supply can be reduced by applying a lower voltage to the power cord. An adjustable transformer called a variac can do this. This trick has a well-deserved reputation for destroying tubes, though. The bias must be adjusted to match the tube operation to the reduced plate voltage.

Adjustable amps have some provision for tweaking these settings. That's why they're intended for use by engineers. Most of us don't know how to set up power tube bias, and the warrantee costs could get out of hand if that was left up to the users.

One issue that I haven't seen mentioned a lot is the speaker. A heavy-duty 50 Watt speaker is not going to sound good if you only feed it 5 Watts. A speaker that sounds good at 5 Watts is not going to last very long when driven by 50 Watts. I keep using the 5 and 50 examples because the 5 Watt output will produce half of the sound pressure level of the 50 Watt output, into the same speaker. A speaker chosen specifically for the 5 Watt amp is likely to be more efficient, so that combination might actually be equally loud as a 50 Watt setup.

On the amptone site, there's a page about the Zeeta amp... http://www.amptone.com/mavenpealzeeta.htm ...which takes the high-quality approach to extreme levels. This helps to mask the fact that a plain 5 Watt amp costs about the same to build as a 50 Watt amp.
Lordathestrings
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#4

The voltage from the power supply can be reduced by applying a lower voltage to the power cord. An adjustable transformer called a variac can do this. This trick has a well-deserved reputation for destroying tubes, though. The bias must be adjusted to match the tube operation to the reduced plate voltage.

Adjustable amps have some provision for tweaking these settings. That's why they're intended for use by engineers. Most of us don't know how to set up power tube bias, and the warrantee costs could get out of hand if that was left up to the users.

One issue that I haven't seen mentioned a lot is the speaker. A heavy-duty 50 Watt speaker is not going to sound good if you only feed it 5 Watts. A speaker that sounds good at 5 Watts is not going to last very long when driven by 50 Watts. I keep using the 5 and 50 examples because the 5 Watt output will produce half of the sound pressure level of the 50 Watt output, into the same speaker. A speaker chosen specifically for the 5 Watt amp is likely to be more efficient, so that combination might actually be equally loud as a 50 Watt setup.

On the amptone site, there's a page about the Zeeta amp... http://www.amptone.com/mavenpealzeeta.htm ...which takes the high-quality approach to extreme levels. This helps to mask the fact that a plain 5 Watt amp costs about the same to build as a 50 Watt amp.
Lordathestrings
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Am I allowed covet that amp? I take it that with an amp like that you don't need to be an engineer to adjust the settings. So my question is, how hard would it be to replicate that control on other amps? I mean, does the whole internal structure need to be changed? This amp seems to be a very high end amp, but I assume you could cut corners and make a cheaper amp that still had similar functions.
"Dozens of people spontaneously combust each year, it's just not that widely reported".

#5

Am I allowed covet that amp? I take it that with an amp like that you don't need to be an engineer to adjust the settings. So my question is, how hard would it be to replicate that control on other amps? I mean, does the whole internal structure need to be changed? This amp seems to be a very high end amp, but I assume you could cut corners and make a cheaper amp that still had similar functions.
"Dozens of people spontaneously combust each year, it's just not that widely reported".

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Any project involves making some choices along the way from concept to final product. Inevitably, something gets traded off. These guys (supposedly) sacrificed price-point for 'perceived quality'.

I emphasise the word perceived here, because they've made at least one choice that sounds like a good idea, but doesn't do anything useful, and actually wastes money that might better have gone towards a real benefit. I mean, of course, the 10 guage steel chassis. A brutally strong chassis is very expensive to make, but as anyone who has any vibration testing experience (like a long road-trip), can tell you, they would have been much wiser to use lighter guage steel, and apply the cost savings toward shock-mounting the chassis.

Which raises two points to ponder.

1) When you spend your hard-earned cash for something, you're expressing confidence in the design and functionality of that item.

2) How much you spend is directly proportional to your agreement with the manufacturer.

In this case, such an obvious lack of understanding the cost/benefit of their mechanical choices reduces my confidence in their electronic design.

Informed consumers understand that there is no such thing as a 'no compromise' product. The key to purchase satisfaction is to choose the combination of compromises that you think you would have made if you had been consulted during the design and production process.

In some cases, that means trusting someone elses judgement. That's where reviews come in handy.

BTW, you are expected to covet that amp! Covet translates into sales!
Lordathestrings
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#6

Any project involves making some choices along the way from concept to final product. Inevitably, something gets traded off. These guys (supposedly) sacrificed price-point for 'perceived quality'.

I emphasise the word perceived here, because they've made at least one choice that sounds like a good idea, but doesn't do anything useful, and actually wastes money that might better have gone towards a real benefit. I mean, of course, the 10 guage steel chassis. A brutally strong chassis is very expensive to make, but as anyone who has any vibration testing experience (like a long road-trip), can tell you, they would have been much wiser to use lighter guage steel, and apply the cost savings toward shock-mounting the chassis.

Which raises two points to ponder.

1) When you spend your hard-earned cash for something, you're expressing confidence in the design and functionality of that item.

2) How much you spend is directly proportional to your agreement with the manufacturer.

In this case, such an obvious lack of understanding the cost/benefit of their mechanical choices reduces my confidence in their electronic design.

Informed consumers understand that there is no such thing as a 'no compromise' product. The key to purchase satisfaction is to choose the combination of compromises that you think you would have made if you had been consulted during the design and production process.

In some cases, that means trusting someone elses judgement. That's where reviews come in handy.

BTW, you are expected to covet that amp! Covet translates into sales!
Lordathestrings
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New Member

Joined: 09/27/01

Posts: 2

Variacs, ruined tubes and low wattage amps

Lordathastrings, One other negative point about using a variac to lower the line voltage is that the filament/heater voltage goes down right along with the line voltage, and tubes do not like being operated without the heaters boiling off a fresh supply of electrons... :eek:

I'll second or third you on the low wattages. I've got a plexi-ish amp I built (based off the November lite at http://www.ax84.com with a push-pull 6v6 power amp section that I figure puts out something like 15-18 watts. It's WAAAYYYY too loud with a 2x12 cabinet for playing in the house. I'm thinking of converting it to something like 2-3 watts, maybe using the 6AQ5 or a 6v6 in a single ended setup with a lowered power supply voltage. I've heard that 6k6's will sub for 6v6's and are a (something like) 3-5 watt tube if memory serves.

You know another thing I rarely see mentioned is the efficiency of the speaker. some speakers put out >100dB with 1W. Others put out about 90dB. That's a BIG difference. I'd like to get/build a low wattage amp, paired with a low wattage and slightly inefficient speaker that has the tonal characteristics of a classic Celestion so that I can drive the bejesus out of both the power amp AND the speaker, and not really have the neighbors getting bent.

Just my $0.03.

geoff

{URL code repaired 15 Oct.}

[Edited by Lordathestrings on 10-16-2001 at 12:33 AM]
What if the hokey-pokey really _IS_ what it's all about?

#7

Variacs, ruined tubes and low wattage amps

Lordathastrings, One other negative point about using a variac to lower the line voltage is that the filament/heater voltage goes down right along with the line voltage, and tubes do not like being operated without the heaters boiling off a fresh supply of electrons... :eek:

I'll second or third you on the low wattages. I've got a plexi-ish amp I built (based off the November lite at http://www.ax84.com with a push-pull 6v6 power amp section that I figure puts out something like 15-18 watts. It's WAAAYYYY too loud with a 2x12 cabinet for playing in the house. I'm thinking of converting it to something like 2-3 watts, maybe using the 6AQ5 or a 6v6 in a single ended setup with a lowered power supply voltage. I've heard that 6k6's will sub for 6v6's and are a (something like) 3-5 watt tube if memory serves.

You know another thing I rarely see mentioned is the efficiency of the speaker. some speakers put out >100dB with 1W. Others put out about 90dB. That's a BIG difference. I'd like to get/build a low wattage amp, paired with a low wattage and slightly inefficient speaker that has the tonal characteristics of a classic Celestion so that I can drive the bejesus out of both the power amp AND the speaker, and not really have the neighbors getting bent.

Just my $0.03.

geoff

{URL code repaired 15 Oct.}

[Edited by Lordathestrings on 10-16-2001 at 12:33 AM]
What if the hokey-pokey really _IS_ what it's all about?

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Posts: 6242

I'm accumulating bits and pieces to build a single tube power stage around a 6AQ5A. I'm contemplating some kind of selector to chose between a solid state rectifier bridge and a 6X4 tube rectifier.

The neat part of using a 6X4 as the power supply rectifier is the fact that the heater filament is separate from the cathode. This allows using a separate (filtered DC?) heater supply.

A nice feature of the 6AQ5A is the nearly linear relationship between plate voltage and cathode bias voltage. I think if I make judicious choices in plate and cathode resistor values, I can set up a Class A self-bias circuit that will perform well over a wide range of line voltages.

I contemplate having a fixed 6.3 VDC heater filament supply and a variable plate supply.

I'll be using a Peavey Rock Master preamp with whatever power amp I come up with. This gives me the option of using either a 1 Volt or 3 Volt nominal drive level to the power stage.

I have an RCA Electron Tube Handbook (circa 1953) which shows the 6K6GT in a single-tube cathode bias circuit rolling off from 3.4 Watts at 250 Volts on the plate, to 0.35 Watts with 100 Volts on the plate. The bias values track as a percentage of the plate voltage (-18V & -7V). I don't know if the sound would be noticeably different from the beam power tubes or not. Looks good!

I chose the 6AQ5A simply because I have a bunch of them, and I can easily accomodate a 6V6 by installing a larger socket.

Its so nice to hear from someone who knows tubes!

[Edited by Lordathestrings on 09-26-2001 at 11:28 PM]
Lordathestrings
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#8

I'm accumulating bits and pieces to build a single tube power stage around a 6AQ5A. I'm contemplating some kind of selector to chose between a solid state rectifier bridge and a 6X4 tube rectifier.

The neat part of using a 6X4 as the power supply rectifier is the fact that the heater filament is separate from the cathode. This allows using a separate (filtered DC?) heater supply.

A nice feature of the 6AQ5A is the nearly linear relationship between plate voltage and cathode bias voltage. I think if I make judicious choices in plate and cathode resistor values, I can set up a Class A self-bias circuit that will perform well over a wide range of line voltages.

I contemplate having a fixed 6.3 VDC heater filament supply and a variable plate supply.

I'll be using a Peavey Rock Master preamp with whatever power amp I come up with. This gives me the option of using either a 1 Volt or 3 Volt nominal drive level to the power stage.

I have an RCA Electron Tube Handbook (circa 1953) which shows the 6K6GT in a single-tube cathode bias circuit rolling off from 3.4 Watts at 250 Volts on the plate, to 0.35 Watts with 100 Volts on the plate. The bias values track as a percentage of the plate voltage (-18V & -7V). I don't know if the sound would be noticeably different from the beam power tubes or not. Looks good!

I chose the 6AQ5A simply because I have a bunch of them, and I can easily accomodate a 6V6 by installing a larger socket.

Its so nice to hear from someone who knows tubes!

[Edited by Lordathestrings on 09-26-2001 at 11:28 PM]
Lordathestrings
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Heh... I only know enough about tubes to be dangerous... :D

Have you ever visited http://www.duncanamps.com ? He's got some really cool free software. A tube database, a tone stack simulator, and a power supply designer/simulator. Tons of good stuff on Spice modelling tubes and such.

geoff
What if the hokey-pokey really _IS_ what it's all about?

#9

Heh... I only know enough about tubes to be dangerous... :D

Have you ever visited http://www.duncanamps.com ? He's got some really cool free software. A tube database, a tone stack simulator, and a power supply designer/simulator. Tons of good stuff on Spice modelling tubes and such.

geoff
What if the hokey-pokey really _IS_ what it's all about?

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Great Site!!

Tray Kewl! I like that Blues 112 amp.

I gotta figure out how to include drawings and schematics in these posts!
Lordathestrings
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#10

Great Site!!

Tray Kewl! I like that Blues 112 amp.

I gotta figure out how to include drawings and schematics in these posts!
Lordathestrings
Guitar Tricks Moderator

http://www.GuitarTricks.com - Home of Online Guitar Lessons