Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: scstill on January 11, 2025, 01:00:01 pm
-
I am in a new project converting Hammond AO-39 to 18W Marshall Lite (Sluckeys)
The 18w Marshall specs a 500uf/50w cathode bypass cap for the EL84 push/pull tubes (I will use 6BQ5), which I need to buy (likely will be 470uf/50w)
Why is the capacitance so seemingly high here?
Is low impedance/ESR important in this application?
-
please post schematic or link to schematic that shows that large of an EL84 K cap also you do know 6BQ5 is the same tube as EL84 right
-
the Marshall 18w schematic we have on file shows 50-500uf. I never noticed that before and now I too wonder why the K cap has that shown. The Dual lite and my 18w TMB are using 47-50uf 100v caps think.
Interestingly the Ceriatone 18W Marshall uses a 500uf/50v k cap.
A larger value cathode capacitor can generally produce a warmer, slightly more compressed tone, while a smaller value may result in a brighter, more articulate sound
-
I remember that issue when building an 18 Lite. It seems that everything from 50-500uf has been used on versions of the 18W. I think Steve used 100uf on his first one :dontknow: He has a schematic for the Brown Note 18 which uses a 500uf.
My ears are so bad that when experimenting I heard no difference from 25-100uf - can't recall if I tried 500uf.
-
after many caps in self biased proto-types, 47uF is where I landed, for each tube, along with it's own Resistor. IMHO anything above 100uf is just fairy dust. That said here's a snippit from a quad EL-84 that used 25uF on one side and 470uF on the other
-
The original used a 50µF. I think those big numbers came from 18W.com. Those guys did a lot of experimenting with the 18W amp. Most of the variations of schematics originated at 18W.com.
Here's the original schematic...
-
https://el34world.com/charts/Schematics/files/Sluckey/Marshall_18w.pdf
https://sluckeyamps.com/hammond/hammond_ao39.pdf
I've never built a Marshall-type amp and wonder how much of its voicing depends on the rather large bypass cap. Google AI says "The value of the bypass capacitor can significantly impact the sound quality, with larger values generally providing more gain and a "fuller" tone, while smaller values can result in a more controlled and defined sound."
-
providing more gain and a "fuller" tone,
from my experiments, after 47uF you're fully bypassed hence no more "gain". 47uf is where pretty much all frequencies in "guitar range" are about equal in amplitude
easy to test;
start with a 10uf cap
sig gen input n scope at speaker
inject an 80hz signal, measure amplitude
inject a 120hz signal, measure amplitude
keep going til ~~5KHZ
change cap, repeat
-
Shoote easier yet I could drag my 18w TMB out of the closet and tack one in there and see how it sounds.. I have bionic ears now though so I wouldn't expect much, but who knows.. :dontknow:
-
A couple of things to bear in mind -
1/ With push pull shared bypass, the bypass cap only does anything useful at high signal levels, in the class B area of operation.
In the class A area, the 'equal but opposite' signals at the cathodes cancel each other out. Mostly.
The point being that it'll tend to be difficult to hear the effect of changing the value of a shared cathode bypass cap. It may tend to be most apparent when the amp is pushing out its max power / being overdriven.
2/ EL84 have perhaps at least twice the mu of most other output valve types we use. Cathode impedance is related to 1/mu, so for the same break frequency, the cap will need to be at least twice the value typically used.
Combining 1&2, yes, as the cathode impedance and resistance values are perhaps half the level typical with other valve types, the ESR value of a bypass cap will tend to be more significant.
-
I have bionic ears
I put that "test" there since someone posted "more gain"
the test "visualizes" "gain" across an audio spectrum, proving/disproving "more gain"
not really meant as a hearing test, more like "mojo" verification :icon_biggrin:
-
Another question on the 18w lite
for the two stage 12ax7 pre amp, does it matter if the plate voltage is split after a single 100k resistor (per schematic) or before with two 100k resistors for each plate?
schematic in reply #6
-
for the two stage 12ax7 pre amp
it's no longer 2, they are both in parallel making it 1 gain stage
-
Another question on the 18w lite
for the two stage 12ax7 pre amp, does it matter if the plate voltage is split after a single 100k resistor (per schematic) or before with two 100k resistors for each plate?
schematic in reply #6
If I understand your query correctly, the 280V HT node is a node, classic linear circuit analysis assumes one point on a node is identical to any other point.
The only node where that falls down is ground / circuit common / 0V, where micro impedances and current flow can cause hum loops.
Sorry if I've got the wrong end of the stick :icon_biggrin:
-
Finished the build but now have a perplexing issue
The PI (middle of layout pic) is not conducting as 0v at the cathode and nearly full HV at plate
The 12AX7 PI is good in another amp (in a preamp position).
I think it is related to the heater voltage when measured across heaters get 0v
but when measuring each to ground get 3.1v (that seems strange).
The heater is tapped from the preamp tube (upper right side of pic) which measures fine at 6.3v as well power tubes heater is correct
The twisted red/black heater wire continuity checks fine. Thinking of replacing it anyway
Hoping you might some thoughts??
Some hand written measured voltages on the schematic (sorry for in work poor quality don't look at the typed voltages)
-
OK Problem solved.
I jumpered the black wire of the red/blck twisted and got full heater voltage and sound
67v at cathode and 196v at plate
But have a problem in the volume/tone area that I need to look at
-
So I finished this project (Hammond AO-39 to Marshall 18w lite) and it sounds pretty good.
I built from Sluckeys 18w lite (schem attached) except I stayed with the Original 5U4 power supply vs 5Y3 in Sluckeys
However, I needed to replace the 12AX7 PI with a 12AU7 because the high gain was causing a lot of squealing.
This is essentially the same Marshall 18w schematic in the EL34 library, so I wonder why I needed to drop the tube down to a 12AU7 to get it to should decent?
-
do your VDC reading match up?
squealing isn't always "too much gain" it could be something as innocent as lead dress, a scope will help there.
another quick "gain" check, put the AU in V1 the AX back in the PI, anything change?
otherwise, be happy, play it like you built it :icon_biggrin:
-
The layout isn't ideal for the degree of gain available, OT wiring seems too close to the input stage etc.
-
The DC voltages on the PI:
12ax7 are 198v plate 67v cathode 65v below 820 cathode resistor
12au7 are 146v plate 101v cathode 99v below 820 cathode resistor
Best sound I get is with 12ax7 in preamp and 12au7 in PI all others have squealing when adjusting vol & tone (med to max levels)
Relative to tubes being close to OT, since the preamp tube is in original Hammond AO39 position it likely is ok, it must be the PI tube as that was added (recall the original Hammond inverted phase with a transformer prior to the PreAmp). I tried a PI tube shield which did not help. I don't readily see a lead dress improvement...
Part of my dilemma with layout is that Sluckey built this with the PI in the same position.
https://sluckeyamps.com/hammond/hammond.htm
-
I think this may have found me something to do with the 330uf lytics that have been just sitting there.... :icon_biggrin:
-
Part of my dilemma with layout is that Sluckey built this with the PI in the same position.
look at the proximity of those PT wires on his compared to those on yours near the PA tubes, the "cleanness" around the pre, he moved the "common ground" you have parked next to V1 Ground isn't as "clean" as you'd think sometimes. just a change of 1/4", can be the difference between inductive coupling.
these are the type "issues" where a good scope jockey can find what the shot-gunners miss
-
I think this may have found me something to do with the 330uf lytics that have been just sitting there.... :icon_biggrin:
Are you referring to the old disconnected cans? Each holds 2x30uf so maybe you mean 4x30uf rather than 330uf. I Did not realize that could have an impact. I have used this approach in the past to preserve the original look. Never thought to gut the old lytics. What might be the reason for these unconnected caps affecting the sound?
-
look at the proximity of those PT wires on his compared to those on yours near the PA tubes, the "cleanness" around the pre, he moved the "common ground" you have parked next to V1 Ground isn't as "clean" as you'd think sometimes. just a change of 1/4", can be the difference between inductive coupling.
these are the type "issues" where a good scope jockey can find what the shot-gunners miss
I took up your suggestions on ground and PT wires, see revision, although the Hammond had the PT wires in that location. Also moved the HV OT blue wire. It did clean up the pot squeal, Thanks. But the sound out of the speakers is very poor with the 12ax7 as PI.
Think I will take up your suggestion and play it with the 12au7.
Not sure that either way sounds like a Marshall though. Maybe I need a bigger room/house...
Its really loud (and running though 2x12). I like it even more when I add power soak between speaker and amp.
Maybe l'll look into getting a scope. Recommendation?
-
Hi scstill
Maybe l'll look into getting a scope. Recommendation?
Have a look at: https://el34world.com/Forum/index.php?topic=32338.0 (https://el34world.com/Forum/index.php?topic=32338.0)
There are other discussions on this topic on the forum as well. Look up "scope recommendations" :laugh:
I have a Rigol DS1102CS (100Mhz but 50MHz is sufficient) This product comes out under various brands. Easy to use, if you are struggling, just push the "Auto" buttom :icon_biggrin:
Typical of this range, the price won't break the bank. An oscilloscpoe is a GREAT INVESTMENT esp if it has the "Maths" Option, ie ability to do Vrms, Vpk, Frequency, etc.
Kind Regards
Mirek
-
Another question, the original Hammond AO39 came with mismatched 6BQ5's; one looks like a Sovtek and the other a Sylvania. Is this why I am getting different plate voltages (320/318) or is this just natural variation? any other concerns with this mismatch? Could this be a factor in the Poor sound with the 12AX7 PI??
I tried a matched pair of EL84's but couldn't tell the difference in sound, but didn't measure the voltage before putting on the plexiglass bottom.
-
Here's a pic of sluckey's AO-39 conversion to 18Watt Lite IIB before he replaced the preamp with the Vox AC-15 preamp. Looks very similar to your conversion.
https://sluckeyamps.com/hammond/18Watt_Lite_IIB.jpg
-
Poor sound
maybe clarify that phase like "sounds dark n muddy" "Not much dynamic range" "no real highs"
otherwise i'm hearing an 'ol Black man on a ramshackle porch in Kentucky, bringing tears to my eyes as he plays Memphis blues :icon_biggrin:
-
I think this may have found me something to do with the 330uf lytics that have been just sitting there.... :icon_biggrin:
Are you referring to the old disconnected cans? Each holds 2x30uf so maybe you mean 4x30uf rather than 330uf. I Did not realize that could have an impact. I have used this approach in the past to preserve the original look. Never thought to gut the old lytics. What might be the reason for these unconnected caps affecting the sound?
No I am speaking about cathode bypass capacitor value and try the 330s out in my 18 watt.
-
Is this why I am getting different plate voltages (320/318) or is this just natural variation?
Uh those plate voltages are the same within 0.1%, jeez how much more perfection are you expecting? :laugh:
-
I think this may have found me something to do with the 330uf lytics that have been just sitting there....
...I am speaking about cathode bypass capacitor value and try the 330s out in my 18 watt.
I'm sorry but I'm not following.
Are you suggesting that the 500uf bypass be change to 330? or something else?
The 500uf bypass is in the original Marshall design
Do you have a link to your 18watt design?
-
Another question, the original Hammond AO39 came with mismatched 6BQ5's; one looks like a Sovtek and the other a Sylvania. Is this why I am getting different plate voltages (320/318) ...
Say I have 2 tires for my car. One is a Bridgestone, and the other is a Pirelli. What does that fact tell you about their air pressure & tread depth?
As you seemed to gather at the end of your question, no matter who made a tube, individual tubes may have same/similar characteristics, or measurably-different characteristics. Usually, they're all over the place even if you have many tubes from a single manufacturer.
-
My two favorite tire brands :-)
The responses in this tend to speak to matched tube pairs maybe not being that critical (within reason).
And not a source of my squeal issues with 12ax7 in PI
-
Your layout has a dangerous feature: the wires from power tubes' anodes to the OT. They carry hundreds Volts of AC and are quite close to circuits having millivolts for full power, so that even small stray capacitances can form positive feedback. At least lead then close to each other so that the signal leakages cancel (they are opposite phase). Put them away from sensitive circuits. In one my constructions long ago, I was forced to shield them, even. It is a pity that BillM's site is gone; I believe that one his mod of Blues Jr to improve stability addressed exactly this - mode the wires closer to chassis. Good luck
-
Your layout has a dangerous feature: the wires from power tubes' anodes to the OT. They carry hundreds Volts of AC and are quite close to circuits having millivolts for full power, so that even small stray capacitances can form positive feedback.
Yeah those coupling caps are flying an inch or so above the HV...
-
Just some suggestions... Put a shield on the preamp tube even if it means replacing the socket. There are a lot of long lead components around the PI tube. Maybe rip it all out, think about it for a while, and rewire. Maybe put the input cap very close to the volume control and run shielded cable to PI pin 2.
Also, tell us everything you know about that OT. I know it's not stock. It comes from a later time. Where did it come from and what are the specs?
-
Just some suggestions... Put a shield on the preamp tube even if it means replacing the socket. There are a lot of long lead components around the PI tube. Maybe rip it all out, think about it for a while, and rewire. Maybe put the input cap very close to the volume control and run shielded cable to PI pin 2.
Also, tell us everything you know about that OT. I know it's not stock. It comes from a later time. Where did it come from and what are the specs?
The OT came with the chassis and seemingly was installed with attention to detail so I figured it worked in the organ. But being suspicious, I measured before the build got a voltage ratio of 50 for impedance of 2500 which at 4ohm load is 10k which I thought is close enough to the 9k the 6BQ5's want.
I have tried the pre and PI shield but didn't get much difference.
Have also thought to move the coupling cap to reduce the HV run.
-
Issues of squeal (fixed) and poor sound (Like the speakers are distorting but in a awful way) happen at high volume which irritates my family (and my ears too)..
So I tried a speaker soak and discovered that the amp sounds very good even dimed with either 12au7 or 12ax7 in PI. Could this point at the speakers maybe not being able to handle its max drive? running into a Silvertone 1484 (two 6L6 in PP) original set of Jensen 2x12s (4ohm load)?
-
Could this point at the speakers
roll in some new/different ones n test
guessing the power soaker is taking 1/2 the power from the speaker, so test new speaker with/without
happen at high volume
another reason a scope come in handy, the amp could be creating the Awful sound, or the speaker, or both.
-
I think this may have found me something to do with the 330uf lytics that have been just sitting there....
...I am speaking about cathode bypass capacitor value and try the 330s out in my 18 watt.
I'm sorry but I'm not following.
Are you suggesting that the 500uf bypass be change to 330? or something else?
The 500uf bypass is in the original Marshall design
Do you have a link to your 18watt design?
Sorry if you read above I stated in reply #2 that the original has a 500uf cap. I am merely stating that I have some unused 330s that I will probably try in my VHT 18W Standard and see how they sound. Nothing more nothing less. In fact it could be totally ignored if you wish.