Marshall clone issue

[rdrgtr]

Just finished this amp.  It is a 50w jcm 800 clone.  I have a B+ of 465vdc across the plates.  I know, probably a little high.  This amp checks out ok, but the audio is a little spikey for my taste.  No real bottom to the sound.  It does have a master volume but it doesn,t break up very well.  I measured 318 vdc hitting the 100k resistors in the first preamp section.  Do I need to bring this voltage down?  I set the bias at 30ma.  Any suggestions would be great.

[stratele52]

B + 465 volts  is good for EL34 and 30 ma bias , good too. What do you mean " spikey". I'm french speaking and Google translation does not know this word.

[tubenit]

Can you post the voltages actually on the preamp tube plates? 

By "spikey", I am presuming you mean somewhat harsh sounding treble?  If not, what do you mean?

And did you change anything from the original JCM800 amp schematic?

With respect, Tubenit

[rdrgtr]

Yea, spikey meaning too much treble.  I did follow the jcm 800 schematic to a "t" to start with.  The amp is quiet and the tone controls work right.  After I posted this last night I did a little research.  One complaint about the jcm 800 is too much treble at low volumes.  Also the buzziness that comes with celestion 75s.  I am suffering from both.  I would like to have more gain as well.  I am going to tweak with it but want to make sure that my voltages are right.  Any input would be great.

[rdrgtr]

On my first 2 plates (pins 1 and 6) of my 12ax7 there is around 240vdc on each one.  Is that a little high?

[tubenit]

On my first 2 plates (pins 1 and 6) of my 12ax7 there is around 240vdc on each one.  Is that a little high?

With 12AX7 tubes I prefer voltages around 165v most of the time and work towards 145-165v range. There are some exceptions to that but even something like a Fender SR or a Dumble amp only has 185-205 range.

So, yes, I would say that's high and could contribute to the spikey treble tone.

You can try paralleling the treble cap with other caps and experiment with higher values such as a 680p or 820p also.

with respect, Tubenit

[rdrgtr]

What would be the down side of putting in a 200k instead of the 100k resistors on the 12ax7 plates?  Would it be better to drop the voltage further upstream at the filter caps?  Any thoughts?

[Matty_V]

Yes it would be better to drop the voltage further upstream. Those are your plate load resistors. Upping them to 220K will change the gain structure of the tube. You could always try it and hear for yourself.

[stratele52]

I'm glad to read I'm not the only one who don't  understand the word "spykey"

IMO almost ( all ? ) Marshall   have "lots " of treble, are spykey.

[stratele52]

Try a "hotter" bias like 35 ma

Cold bias give thin sound

[Megachunk]

Are you using a bright cap? Maybe drop the value or try it without all together.

Just my 2 bits.

Megachunk

[Deric]

Which JCM800?  2204?  Have a schematic and/or pics?  What speakers are you using?

2204's were designed to use Celestion G12-65's which are not as bright as most other Celestions.  The first time I had a chance to really play with a 2204 I HATED it.  Was using a 1960a cab with 12-75's.  Was WAY too bright for me.  A friend let me borrow a 4x12 cab loaded with 12-65's and it was AWESOME!

[stratele52]

rdgrt , what kind of signal capacitor do you use ?   Mallory Orange drop ? Or another brand ?

And also the value of those cap can make big diiference

Like on V1 pin 1, pin 6 , if you put a 0.002 Mfd it will sound brighter than a 0.02 Mfd

And I agree with Deric , which schematic ?

And what is the brand name of your output transformer ?

[rdrgtr]

I got it figured out.  I lowered the plate voltage on the first preamp stage (12ax7).  I cut out the .001 brite cap.  I then removed the 470k and 500p bypass cap in the first preamp stage and replaced with a straight 68k resistor.  Those 3 things made a HUGE difference in the amp.  I used and old power transformer that was giving me a very high B+.  My el34s didn't seem to mind to much but the high voltage on my preamp plates was causing them to sound " spikey" ( I did not make that word up).  I replaced the 10k voltage drop resistor with a 40k.  She got real sweet after that.  Why would someone have to add an exrta tube in a jcm 800 to get gain?  She has triodes o-plenty and plenty of places to get gain. Thanks guys for the help. 

[tubenit]

Good job!  Thanks for sharing the resolution and I'm glad you're happy with it!

Congrats!  With respect, tubenit

[PRR]

> What do you mean "spikey". I'm french speaking and Google translation does not know this word.

It may not translate well. In this case, it helps to put "spikey" into Google (english) Images and see what pictures go with the word.







The root-word is "spike". "Spikey" is, in typical english new-word tradition, the adjective for "covered with lots of spikes".

It isn't a good word for sound quality, but "Writing about music {or sound} is like dancing about architecture."

[stratele52]

Congratulation rdgrt good job. You'll be an expert in Marshall mods.

Thanks PRR for description of spikey

[phsyconoodler]

The JCM800 was designed to play live at high volumes into a 4-12 and sometimes two 4-12 cabinets.They cut through a mix nicely that way.
  But players nowadays play often sitting in their basements and living rooms at much lower volumes.
Here is a layout that should give you the gain you want.I have used this in many amps with great success.

[shortfuse]

rdrgtr
Out of curiosity what were the plate voltages on the preamp tubes when you finished?

Tubenit
Very interesting comment on the pre-amp plates that you try to obtain.  I always thought 180 - 190 wee normal.  That may be the problem with one of the amps I built and am going to play with it this coming weekend.

[rdrgtr]

No problem.  The plates in the first two stages were 181 and 176 if I remember correctly.