Can an ez81 cause compression in the preamp?master set low?

puddlefish

Member
Messages
190
I have a custom "conversion" amp with a pair of el84's,2-12ax7's and an ez81/6ca4 rectifier.

It has a master volume and gain control in the preamp for overdriven tones at less than full volume.

Can the rectifier impart some compression with the master set low and the preamp set near full?

FWIIW the plate resistor's on gain stage 1 and 2 the values are 100k and the cathodes 2.7k and 2.2k respectively(not sure if those high values could factor in,5e3 has 100k plate but 820 on the cathodes). I checked the voltages,swapped all tubes but still it compresses allot with the pregain at/past %75.

It isn't terrible but it's a bit more than I would like.
 

puddlefish

Member
Messages
190
Come to think of it I once owned a Univox u45b(recapped completely with new/same value units) with the same ez81 rectifier and that amp compressed allot as well,very noticeably at full volume.It's hard to know if the rectifier was the main reason but even the clean/slightly dirty tones were really really smooth,maybe a bit compressed,perhaps a bit sluggish.
 

Laurent Brondel

Silver Supporting Member
Messages
2,781
Can the rectifier impart some compression with the master set low and the preamp set near full?
I wouldn't think so, there is really no discernable difference between tube or SS rectification at low to medium volume. It's only when operating the amp at or near maximum volume that some tube rectifiers present some kind of latency. Gain at 75% sounds to me like preamp distortion.
 

Blue Strat

Member
Messages
30,182
I wouldn't think so, there is really no discernable difference between tube or SS rectification at low to medium volume. It's only when operating the amp at or near maximum volume that some tube rectifiers present some kind of latency. Gain at 75% sounds to me like preamp distortion.
Agreed. The biggest unknown here is this "conversion amp". We have no idea what it really is and can't reasonably comment without knowing more.
 

puddlefish

Member
Messages
190
" there is really no discernable difference between tube or SS rectification at low to medium volume"

That's what I thought,the guy who built it said that it can affect low volume by making it more "slow" on attack.My follow up question was about bias.I since rebiased both gain stages closer to 1.3k and still if is compressed, a bit like distortion blocking.
 

puddlefish

Member
Messages
190
Agreed. The biggest unknown here is this "conversion amp". We have no idea what it really is and can't reasonably comment without knowing more.
I'm not able at the moment to post the schematic(terrible with computers).I can live with it for now,just trying to get it to sound as good as possible eventually.
 

Badside

Member
Messages
1,411
I have a custom "conversion" amp with a pair of el84's,2-12ax7's and an ez81/6ca4 rectifier.

It has a master volume and gain control in the preamp for overdriven tones at less than full volume.

Can the rectifier impart some compression with the master set low and the preamp set near full?

FWIIW the plate resistor's on gain stage 1 and 2 the values are 100k and the cathodes 2.7k and 2.2k respectively(not sure if those high values could factor in,5e3 has 100k plate but 820 on the cathodes). I checked the voltages,swapped all tubes but still it compresses allot with the pregain at/past %75.

It isn't terrible but it's a bit more than I would like.
I would assume the voltage dropping resistors would have a LOT more effect than the rectifier tube with the master set really low. Also just generally what voltage is on those preamp plates, the capacitor feeding them B+, etc.

The 5E3 has 820 because it's shared between both triodes. It's equivalent to having a 1.6k on each one.
Using higher values (2.7k and 2.2k) changes the distortion sound a bit, though I wouldn't call it more compressed IMHO but it sure makes a difference. But so does everything else.

What kind of master volume is it running? A PPIMV makes things more compressed IMHO, it helps getting more gain but doesn't necessarily sound better.
 

pdf64

Member
Messages
7,359
Can the rectifier impart some compression with the master set low and the preamp set near full?
The rectifier's sag can only come into effect when signal levels at the power tube grids are such that the power tubes begin to pass more cathode current than their idle level, ie as they move out of their 'class A' area and into their 'class B' area.
The particular point on the various gain controls in the signal path at which that occurs, such as those quoted above, are immaterial.
ie if the power tubes are working 'harder than idle' at that point, then HT sag may occur.
Be aware that HT sag induced signal compression may occur at the plate node, screen grid node, or both.
Note that grid clipping bias shift / blocking distortion can affect any stage, and as it lowers the affected stage's gain, may be perceived as having a compression type effect.
 

Tone_Terrific

Silver Supporting Member
Messages
32,981
The rectifier's sag can only come into effect when signal levels at the power tube grids are such that the power tubes begin to pass more cathode current than their idle level, ie as they move out of their 'class A' area and into their 'class B' area.
At low levels (under 1W) would you expect any audible compression from power supply sag? Seems odd to me.

It isn't terrible but it's a bit more than I would like.
And what if you simply do not turn drive up so high?
Generally, tube preamps sound quite thin, compressed and buzzy unless designed /voiced to be otherwise.
 

puddlefish

Member
Messages
190
I would assume the voltage dropping resistors would have a LOT more effect than the rectifier tube with the master set really low. Also just generally what voltage is on those preamp plates, the capacitor feeding them B+, etc.

The 5E3 has 820 because it's shared between both triodes. It's equivalent to having a 1.6k on each one.
Using higher values (2.7k and 2.2k) changes the distortion sound a bit, though I wouldn't call it more compressed IMHO but it sure makes a difference. But so does everything else.

What kind of master volume is it running? A PPIMV makes things more compressed IMHO, it helps getting more gain but doesn't necessarily sound better.
It's PPIMV.Come to think of it it compresses and squishes only after %75 gain,a bit like a 5e3 does
 

puddlefish

Member
Messages
190
Note that grid clipping bias shift / blocking distortion can affect any stage, and as it lowers the affected stage's gain, may be perceived as having a compression type effect.
Makes sense,I've heard it in some Fender amps
 

puddlefish

Member
Messages
190
And what if you simply do not turn drive up so high?
Generally, tube preamps sound quite thin, compressed and buzzy unless designed /voiced to be otherwise.
? It's a 12-15 watt amp so it's used near or at full volume.It sounds throaty and crunchy like a Vox,not thin or buzzy.I mentioned the compression at low volumes because it will do that at high volumes too when the gain is past %75.If I crank the master and set gain at %75 it's pretty thick and not so compressed,a tiny bit more and it's quite compressed,sort of 5e3-like in that sense.
 

puddlefish

Member
Messages
190
I might be making too much of this,it was just a weird experience to have a master volume amp with only moderate lows go that squishy at low volumes/...again only squishy past %75 on the gain control.I thought maybe there was something wrong with it.
 

Tone_Terrific

Silver Supporting Member
Messages
32,981
moderate lows go that squishy at low volumes
It's a 12-15 watt amp so it's used near or at full volume.It sounds throaty and crunchy like a Vox,not thin or buzzy.
Good.
Most amps reach a point of mush with enough gain. If it is good with the master up to usable level there may be nothing else to do.
You would have to analyze it stage by stage to check/alter the distortion character.
If you have it sounding good with the master up and then it goes to mush as the master is reduced that would be weird.
 

pdf64

Member
Messages
7,359
At low levels (under 1W) would you expect any audible compression from power supply sag? Seems odd to me
Yes, I also wouldn't expect much HT sag in a regular 2 x el84 amp until the signal level was increased to well above half its clean power output.
 




Trending Topics

Top