(Archive) Frequently Asked Questions

[tbeck]

It'd be cool to see a "frequently asked questions" area in the "amp stuff" section for questions like my capacitor question.  A person could just go there and look for that info instead of asking the same questions again. the FAQs could be pulled from this topic area and put in there.  Basic questions on components and maybe common mods could be in there.  The whole thread of the question could be moved there after a week or two on the regular postings. Maybe editing of non-related stuff in the thread.
 Just a thought.

[GroundhogKen]

OK, the "Official" FAQ thread starts here.

DISCLAIMER

Please do not work on tube amps if you do not know how to work on tube amps safely.
 
The following answers to FAQ's are intended for educational purpose and it is strongly suggested to check other sources for additional information and accuracy. The answers are not intended to promote a particular approach to amp building or repair. It is your responsibility to know how to work on your amp safely and correctly.
 
It is highly recommended to read Doug Hoffman's Library of Information first as many answers are listed there as well as other information you will need.  


Ken

[YD]

Maybe something along the lines of pin names for common tube types (12ax, 6l6s, 6v6s, ELs). ie

12ax7
Pin 1 - Plate
Pin 3 - Cathode
Pin 4 - Heater
etc etc..

It is definately easier to understand some posts if you know what someone is talking about when they say something like grid or plate .  Someone might think it's a meal - a plate full of grits.. ok Im done.

[tubenit]

1) Are blue glowing tubes bad?
 
2) Should I form electrolytic capacitors?
 
3) How do I wire my speakers up for the correct impedence?
    (How do I wire up five 8 ohm speakers to get exactly 9.782643 ohms? )
 
4)  How do I get more headroom from my Deluxe Reverb?
 
5)  How do I make my Super Reverb break up earlier?
 
6)  How would I determine the impedence (or winding ratio)  of an unknown
     transformer?
    (I found a transformer in a garbage can in back of a Hardees.  What are its  specs?)
 
7)  How do I start tracing out a problem if the amp is not making any sound?
     (I wired up a plexi and get no sound.  Does anyone know of any issues with the  
plexi design?)
 
8)   Why does my amp hum and how do I fix that problem?
 
9)   My amp has a B+ of 837 volts.  Can I run 6v6 tubes?  What is  the voltage rating of 6V6’s?
 
10)  How do I increase the vibrato on my Princeton Reverb or slow down the vibrato?  

11)  Anybody have a schematic showing how to wire up a  pentode triode? Does it work as well on 6L6’s as 6V6’s?  Can you use them on EL34’s and EL84’s?
 
12)  What are the “best” coupling caps for an amp?  

13)   Can you safely run a mismatched  impedence between an output transformer and speaker or will it fry the output transformer?

14)   Is there some rule of thumb as to how much voltage increase there is with 5Y3, 5U4GT, GZ34 or solid state rectifiers?  

15)   Can I switch my amp from cathode to fixed bias or fixed bias to cathode? What
is the difference in the volume and tone?

16)   What should I do to safely fire up my amp for the first time?

17)    How do you parallel the preamp tube and what does that do for the gain vs. series gain stages?

18)    What are the  differences in tone stacks like Fender, Bassman/Marshall & Baxandall  as far as tone and “insertion loss”?  What is insertion loss?

[tubenit]

19)     What is the best baffle board material and thickness ?

20)      How does someone  apply  tolex and lacquer tweed?

21)      Why use a choke instead of a resistor and how will the tone be different?

22)      Is there  some guideline for the values of a cathode resistor with plate resistor in regards to Gain?

23)       What is the correct way to bias an amp?

24)      My amp has 6L6’s can I use EL34's in a 6L6 amp?   Can I use 12Ax7's instead of 6EU7’s?

25)     How can I find out what each of the pins are on a particular tube like a 12AX7?

[triode]

1) Are blue glowing tubes bad?
 

Not all all, see this link for all the info you need:
http://members.aol.com/larrysb/blue_glow.html

2) Should I form electrolytic capacitors?

Too much heated discussion about this topic, if you are interested in reforming, go read this:
http://www.vcomp.co.uk/tech_tips/reform_caps/reform_caps.htm

3) How do I wire my speakers up for the correct impedence?
    (How do I wire up five 8 ohm speakers to get exactly 9.782643 ohms? )

You can work backwards using the series and parallel formulas for resistive loads. You can also
combine series and parallel arrangements to achieve other impedances. Lets see the formula and then
work a small example...

Rt = the total resistance
R1, R2, R3,...Rn is each speakers impedance (DC ohms, we will use the stated number, even though that is an average)

Series: Rt = R1 + R2 + R3.... + Rn
Parallel 1/Rt = (1/R1) + (1/R2) + (1/R3) + ... (1/Rn)
For the mathematically inclined you will note there is a special case of parallel, namely if you have two speakers (resistors) [of EQUAL impedance] then the total resistance is simply Rt = (R1 + R2) / 2.

Simple example: two 8 ohm speakers, you want 4 ohms: wire them in parallel ( + to + on each speaker goes to + on amp, - to - on each speaker goes to - on amp) to get four ohms.

Simple example2: two 8 ohm speakers, you want 16 ohms: wire them in series (+ of speaker 1 goes to + on amp, - of speaker 1 goes to + on speaker 2, - on speaker 2 goes to - on amp)

More complicated example: you have a bassman with four 8 ohm drivers and you want 8 ohms: wire two in parallel for four ohms.  Wire the other two in parallel for four ohms.  Take those two leads and wire them in series which will give you an eight ohm load.

[triode]

6)  How would I determine the impedence (or winding ratio)  of an unknown
     transformer?
    (I found a transformer in a garbage can in back of a Hardees.  What are its  specs?)

Obviously, if there are any numbers on it whatsoever you should go to Google and see if anything
comes up. However, if you have an unknown tranny in your hands, you can cerainly find the winding ratio
if you like. You will need a good plain old AC signal source capable of putting out 1V RMS AC, and a descent
meter. Take your AC souce and plug it into the suspected primary. Put your meter on it and see if you are reading
1V AC (on the primary, just to make sure, if you are not, adjust your signal source)... now move your meter
over to the secondary of the transformer and take the reading there. Since the input was 1V AC, you can directly
read the voltage ratio here... The turn ratio (N) is equal to the voltage of the primary divided by the voltage
of the seconday. Since you measured both (albeit crudely) you can get a good estimate of the turns ratio.

7)  How do I start tracing out a problem if the amp is not making any sound?
     (I wired up a plexi and get no sound.  Does anyone know of any issues with the  
plexi design?)

Read this entire section here first:
http://www.el34world.com/charts/fenderservice1.htm
http://www.el34world.com/charts/fenderservice2.htm
http://www.el34world.com/charts/fenderservice3.htm
http://www.el34world.com/charts/fenderservice4.htm
http://www.el34world.com/charts/fenderservice5.htm
http://www.el34world.com/charts/fenderservice7.htm

Back already? There is nothing wrong with the Plexi design, it has been in use for many years.
<this section to be added to>

8)   Why does my amp hum and how do I fix that problem?

Your amp can be humming for a number of reasons, the usual suspects are grounds, heater (filament lines)
inducing noise, bad lead dress, bad shielded wire (or none on the input), etc, etc.
Read the amp noises page:
http://www.el34world.com/charts/fenderservice6.htm

[triode]

25)

14)   Is there some rule of thumb as to how much voltage increase there is with 5Y3, 5U4GT, GZ34 or solid state rectifiers?  

Mostly it tends to be "look it up in the datasheet" on this one, although the following is a nice start:

5AR4       -17 volts @ 225 mA
5U4         -44 volts @ 225 mA
5U4GB    -50 volts @ 275 mA
5Y3         -60 volts @ 125 mA

Note, it depends on the current being sinked by the tube. This is also noted in the tube data sheets (RCA
receiving tube manual has charts on 5AR4/5U4GB/5Y3GT).

Note also that not all of them are interchangable, as some have lower current limits than others, and
some (the 5AR4 being an effecient model) will put out a bit more voltage than say a 5Y3GT... always
be sure to check your B+ line after to see if you are putting too much voltage on the rest of the circuit.

[Ken's NOTE]

I use the following rule of thumb:

If you power transformer puts outs Vrms-0-Vrms,
then:

B+ = 1.4 * Vrms---for full wave solid state non-bridge recitifier,

B+ = 1.3 * Vrms---for GZ-34(5AR4) tube recitifier

B+ = 1.25 * Vrms---for 5U4 tube recifier

B+ = 1.15 * Vrms---for 5Y3 tube rectifier.

These are approximate numbers which assume you are drawing a "reasonable" amount of current from the transformer.  The voltages would be significantly higher if no current is drawn--like the when amp is in Standby.

[END NOTE]

[tubenit]

#14 continued .....

To add to triode's rectifier info. A good starting place to know how much increase in voltage with the rectifier tubes would be the following:

5Y3 is 1/2 AC x1.1
5U4 is 1.2
GZ34 is 1.3
solid state is 1.4

[LooseChange]

13)   Can you safely run a mismatched  impedance between an output transformer and speaker or will it fry the output transformer?


It is okay to mismatch no more than 100%.  For example: 8 ohm output matched to 16 ohm or 4 ohm is okay.  This holds true for most commercially sold amps like Marshall and Fender.  Some overbuilt transformers like Hammond can take even a larger mismatch.

[Ken's NOTE]

100% mismatch is usually safe if you running up to the point of maximum undistorted power.  If you plan to turn the amp up into full clipping, proceed at your own risk with a mismatched load--at least with a vintage amp with an original transformer


Regarding impedence when removing two tubes:

Think about the logic of it and you'll always remember the answer.  With half the tubes you have twice the impedance due to only having half of the current pathway.  4 garden hoses have less resistance than 2 garden hoses.  With the 2 garden hoses (tubes) pulled, your resistance doubles to the water flow.  The same with the tubes.  Pull two of four tubes and your impedance (which is AC resistance) doubles.  Therefore, if your primary impedance goes up, you have to raise your secondary impedance to keep the same ratio between primary and secondary, so the speaker ohms doubles too.

[LooseChange]

21)      Why use a choke instead of a resistor and how will the tone be different?

A choke is typically used in more expensive amps.  They cost more than a resistor.  More importantly, they do a better job reducing hum in an amp.  Many say that a choke will make the amp "feel" better when played.

Chokes are used in the power supply.  In most amps they are placed between the power tube supply (B+) and the screen supply. A choke will keep the supply to the screens very stiff.  This is what amp designers want.

[OldHouseScott]

4)  How do I get more headroom from my Deluxe Reverb?

One might start with a more efficient speaker such as a Weber California. After that try a 12AY7 instead of 12AX7 in the preamp positions V1 & V2. Getting into the amp, change the power supply dropping resistors from the 10k/10k values to 1k/4.7k as used in the 40W and 80W Fenders. A change to 6L6's along with an upgraded OT of the appropriate impedance will gain more clean sound at a given volume. After these changes, you end up with basically a Pro or Super circuit.

[tubenit]

5)  How do I make my Super Reverb break up earlier?  

The easiest ways I can think of making this happen are:
-  try a 12AX7 that is known to be a higher gain tube
- use a speaker that will break up earlier. An alnico may be the ticket for this.
   Weber has some excellent speakers that will accomplish (as do some other
    brands)
- lower the V1 cathode resistor from 1.5k & try 1K or 820ohm
-  if you further need to lower voltage to V1 you might raise the plate resistor value
   to 120k-180k range until you get around 150 volts on the plate
- you can experiment with lowering the B+ to the preamp by increasing the
   resistors to 2.2K or 4.7k (between filter cap B & C) and to 1K (between filter cap
  C&D)
-  use a pentode/triode switch so at lower volumes you can drive the tubes  
      more* see post on pentode/triode
-  changing tone stack values to emphasize mids may increase
    the "feeling" of being overdriven sooner
- try changing the V1 cathode cap from 22uf/25v to 47uf/50v this can increase the mids and gain
  a little bit

[tubenit]

9)   Can I run 6v6 tubes?  What is  the voltage rating of 6V6’s?  My PT is putting
      out around 475 volts (or more) and has had 6L6's?
      
 Many 6V6’s will not handle voltages over 420volts.  I have run JJ 6V6’s at 467 volts playing for over 10 hours at that voltage without a problem. Others have reported JJ’s to be a more robust tube and capable of handling higher voltages than other 6V6’s.

[tubenit]

10)  How can I increase the vibrato on my Fender amp (or slow it down)?

"This should be useful for Hoffman AB763 bias vary tremolo amps with 6L6 tubes.
 
http://www.hoffmanamps.com/images/Ab763.gif
 
The Tremolux 6G9-B circuit has a beefier tremolo drive than the brown Vibroverb circuit--and definitely more intense than the Blackface Princeton.
 
Reference the Hoffman AB763 layout.
Locate the 4.7K cathode resistor on on V5 pin 3.
Install a 22uF 50v electrolytic cap in parallel with the 4.7K resistor.  (positive end to pin 3, negative to ground)
This should help even the wimpiest 12AX7s to oscillate.
 
Locate the two 470K resistors that connect to pin 8 of V5.
Change the one that is horizontal on the layout to 220K.
There are two .1 caps that attach to the right end of this 470K (now 220K) resistor.
Change the .1uF cap that goes to ground (the left one, not the one that goes to the intensity pot) to .047uf.
 
http://www.ampwares.com/ffg/schem/tremolux_6g9-b_schem.gif
 
The original Hoffman AB763 is probably fine for 6v6 amps. "
 
To slow down the vibrato change the caps in that section from .02/.01/.01 to .02/.01/.02

[tubenit]

11)  Anybody have a schematic showing how to wire up a  pentode triode? Does it work as well on  6L6’s as 6V6’s?  Can you use them on EL34’s and EL84’s?

       
         There are a number of websites that will show schematics of wiring a pentode/triode switch.  It works great on 6V6’s reducing the volume to 60% with a warmer more compressed sounding overdrive/distortion. My experience and the report of others is that it may not be as dramatic of a tone/volume change with 6L6’s?  Reportedly, it can be used for EL34’s and EL84’s effectively.
        
The grid resistor may need to be increased from 470 ohm to 1-2.2k/3w resistor. The 2.2k resistorseems to contribute to a more compressed tone.

Use a heavyduty toggle DPDT with that can handle the amps/volts.

        Also do a “search” in the current forum posts for pentode/triode.
 
http://www.hoffmanamps.com/Forum/yabb2/YaBB.pl?num=1130445301
 

[tubenit]

12)  What are the best coupling caps for an amp?

This is a totally subjective area. Whatever sounds best to you is the best answer.
*  See other archive question on this issue


"I used to be a pretty big proponent of 715p polypropylene film/foil Orange Drops.

Nowadays I don't think I would build an amp from the ground up exclusively with ODs, and I would certainly never remove a bunch of caps from a vintage amp and blindly install ODs.

Building an amp and swapping the caps yourself is the best solution. I suggest if you build a push pull amp with a long tail phase inverter that swapping the cap at the input to the phase inverter and the caps between the PI plates and the power tube grids will give you a pretty good idea of the difference between ODs and Mallory 150s. What you a mostly comparing is polypropylene vs polyester.

If you don't want to go to the trouble of cap swapping, I'd say Mallory 150s are the safest bet for vintage tone. "

"It depends what you're doing or what kind of sound you're going for. Marshall style circuits sound good with mallory 150's, nice mids, or mustard caps - quintessential Marshall right there, if you can find those. I like the Orange Drop 716's for fender style circuits, nice round bass. Don't forget your ceramics with the fender circuits, they're noisey but add to high end detail. Ampeg stuff sounds good with Orange drops.
I also like using Solens in amps that require a fast responsive power supply but that's more the feel. In fact, if you want a tight loud clean power supply, Hiwatt style, I highly recommend these as your first two caps between the 470 Ohm resistor or the choke, they're super tight sounding/feeling.

To me the Orange drops and the mallory's are the most musical sounding. "

[tubenit]

15) Can I switch my amp from cathode to fixed bias or fixed bias to cathode? what is
      the difference in volume and tone?

Doug's LIBRARY of INFORMATION has alot of information on biasing. Read it!

"At lower volumes you probably won't hear much difference.As the volume levels go up,the cathode biased amp will distort sooner and the bass notes will lose definition in comparison to the fixed bias amp. The fixed bias amp should be louder and cleaner longer,assuming the amp is set up properly"
 
This site has some information about switching from one to the other:
http://www.londonpower.com/cath-fix.htm

[tubenit]

16) How do I safely fire my new amp build up for the first time?

"Start the amp up with no tubes and measure all the voltages first.Especially the filament voltages.Then the B+ at the plates and then the bias voltage.
If those are in line,you will probably have no problems.Recheck the circuit against the schematic and trace it with a highlighter. Make sure all the caps are oriented the right way.Neg. to ground.Check all grounds with your ohmmeter. "

"Start it up with a light bulb current limiter. Saved me a zillion fuses and probably a few transformers too."

"Good idea from loose change. Wire two outlets so that they are in series. You can use a regular duplex outlet and break the contacts between the two and wire them up series. Plug a 100Watt lamp into one and your amp into the other. This way when you power up the amp shares the current with the lamp and is somewhat protected...if the amp has a short circuit the lamp will glow bright and "take the heat". If everything is cool both will be dim. Gerald Weber has it outlined clearly in one of his books"

[tubenit]

17)    How do you parallel the preamp tube and what does that do for the gain vs. series gain  
        stages?

Paralleling a V1 involves connecting the plate to plate, cathode to cathode and grid to grid of a  12A_7 for example. It tends to give about 30% increase in gain at that stage without an increase in idle noise.  The paralleled stage may be a warmer smoother gain  than series gain.

  Gain stages in series give a greater increase in gain but more increase in idle noise.

          Using a 12AX7 as a gain factor of 20, look at the comparison:
          Paralleled V1 is 40 (20+20) x 20 (following gain stage ) for = 800 gain
          Series is 20x 20x 20 for = 8,000 gain.

[tubenit]

18)    What are the  differences in tone stacks like Fender, Bassman/Marshall & Baxandall  as far as tone and “insertion loss”?  What is insertion loss?  

Insertion loss means a loss of gain (which could be the same as a functional loss of volume). There is not a best tone stack, it is a matter of personal taste. Almost any tone stack can be tweaked .

The "traditional" Fender AA763 type tone stack has signficant mid-scooped emphasizing highs and lows. The topology of it tends to be between the first and second gain followed by the volume pot.

The Bassman and Marshall tone stack topology is tends to be between the 2nd and 3rd gain stages with the volume pot between the 1st and 2nd stage.  Marshall tone stack may have more mids (or less bass) than the typical Fender AA763. With the tone stack further away from the input, the tone controls may not have as significant as much of a change in tone as with the AA763 topology (which is closer to the input signal)

The Baxandall tone stack is a versatile one used in some guitar amps and hi-fi equipment. Even though it is two pots, … it can allow closer to a true mid-boost. It may be a great choice for blues or jazz. One website suggests that the Baxandall is a good choice for low to moderate watt amps OR for lower gain amps.  However, some individuals who have used it for larger watt amps or high gain amp express not liking the tone.  The baxandall can be inserted between 1st and 2nd stage gain OR between 2nd and 3rd stage gain.  Reverb can be inserted into the tone stack and the other reverb connection post the volume pot wiper (with the volume pot following the Baxandall)

[tubenit]

#18 continued ........       The 5E3 Deluxe tone stack has very low insertion loss and is easy to wire up. At some level, it tends to be interactive with the volume pot.  I have added reverb by inserting  on the non-grounded 1M volume pot terminal & with the other reverb connection  to  the vol pot wiper to the next tube stage grid. I have used various resistors of 10k-330k between the volume pot wiper and the next grid tube stage.

The Duncan  tone stack calculator can allow one to change components to see the impact on tone.
             http://duncanamps.com/tsc/

This is another website that has some great information and examples of wiring up tonestacks.
              http://amps.zugster.net/articles/tonestacks/index.html

Simply put, .. the topology of where the tone stack is added, the type of tone stack and the component values all play a factor in shaping the tone.

"Western Electric = gain> (vol) gain>cathode follower>tonestack>PI
 
like the 5F6-A Bassman
http://schematicheavencom.secure.powweb.com/fenderamps/bassman_5f6a_schem.pdf
 
 
AB763 = gain>tonestack (vol) >gain>(gain)>PI
 
like the Super Reverb
http://schematicheavencom.secure.powweb.com/fenderamps/super_reverb_ab763_schem. pdf
 
So, tonestack placement, & how the TS is driven.
 
ps.....Another variation of the W.E. = gain>gain>cathode follower>>tonestack>gain>PI
 
Like (my personal favorite) the 5E5-A Tweed Pro
http://schematicheavencom.secure.powweb.com/fenderamps/pro_5e5a_schem.pdf "

Da Geezer

[tubenit]

19) What is the best baffle board material and thickness?

"If it were me, I wouldn't use anything less than 1/2 inch for todays large magnet speakers. I would prefer to see you use 11/16ths or 3/4 inch.

3/8ths is too thin and 5/16 is even thinner, and by the way, 5/16ths is sold at home depot in lew of 3/8ths inch. A lot of the 5/16ths inch is usually 3/8ths finished on one side....

There should be 1/8th inch all around the inside of the baffle mounting space(for expansion and contraction)[IF you are using a separate baffle and grill, or combining them(before the grill cloth)...either way] and I would use 6 bolts, 2-top and 2- bottom, and one in the center of the sides, at the very least. 4 doesn't cut it for strength, rigidity or tone, no matter which cabinet you are building. I would suggest 8, with 2 more in the center of the top and bottom, with the above."

Should be noted that many Fenders used 5/16" plywood. Birch plywood seems to be a reasonable choice.

[tubenit]

20 part A)  How does someone apply tolex?

"I would get two yards of tolex for a twin reverb.I use contact cement for glue.It smells but it works well.The corners take practice to get right. If you take the old tolex off,take note of how the corners are attached. Do not use spray on glues as they don't work at all.They never seem to dry and the tolex will peel off in a day or two and drive you nuts.Let the cabinet dry for 3 full days before attempting to asemble it.You will thank me later."

"As much as you can, try to keep the old tolex intact and use the pieces as templates. Study how Fender formed the corners. The top corners are the most difficult areas to get right but the tolex is very forgiving and flaws aren't as obvious as they'd seem to be because of the texture. If you look at most Fenders, you're surprised to see how sloppy these corners are when you actually look closely. I used both the 3M and the water based contact cement in different places and by far appreciated using the water base more. Buy a couple-six throw-away rollers to apply the cement. A second pair of hands is very helpful to hold up the tolex while you're working to get a corner correctly placed. You'll also want to have a can of wood filler handy to address flaws in the wood where Fender used filler and it has come out with the old tolex. Plan ahead on how you intend to lay on the tolex and align the visually critical areas first."

"A exercise that helped me when I starting out was to use a sheet of 8 1/2" x 11" ink jet copy paper and tape it with masking tape to the side of your amp at the corners just like the normal piece of tolex would fit at that location. That way you can practice with paper and detemine the best way to cut the corners. It's better leave yourself a little extra material than needed when trimming for corners because in my experience when you apply the contact cenment and it drys out the tolex tends to shrink up a bit and the seems can open up."

[tubenit]

22)  Is there  some guideline for the values of a cathode resistor with plate resistor in
       regards to gain?

Generally speaking you’ll find ratios such as 100k (plate) to 1.5k (cathode), or 220k/2.2k or
330k/3.3k, etc……    There are exceptions to this of course.  In other words, they tend to be
a ratio of 100:1 units

            Examples if you change nothing else:
Increase plate resistor value = more gain   Decrease plate resistor value = more headroom
Increase cathode resistor value = more head room   Decrease cathode resistor value = more gain
            
             For Example:  a 100k/ 1.5k has less gain than a 100k/820 ohm

A browner more tweed sound may have 140 volts on a 12A_7 plate and 160 -200volts may have a cleaner sound and more headroom.

[tubenit]

23) What is the "correct" way to bias an amp?

There are differing opinions on this to the point of controversy and individuals wanting to get the "last word on biasing".  I've seen well respected and knowledgeable forum members take an approach of " I bias to what sounds good to my ears without the power tubes redplating" and also others take the strict "70%" biasing rule for fixed biased amps. The 70% rule is related to class AB amps.

This site has good tube data and a biasing "calculator":
http://www.webervst.com/vstbbs/bbs.html

This site has some information about the 70% "biasing rule"
http://www.aikenamps.com/Biasing.html

I have found on my cathode biased amps that they don't bias within 70% but tend to exceed that. I have read that the 70% rule is not as critical on cathode biased amps and also that the tubes in a cathode biased amps tend to draw the current that the want.
Having said that, in a cathode biased amp a 300ohm 7W resisitor will tend to reduce the current more than a 250 ohm resistor.

[tubenit]

24)      My amp has 6L6’s can I use EL34's in a 6L6 amp?   Can I use 12Ax7's instead of 6EU7’s?

Look up a data base such as   http://www.nj7p.org/Tube.php
or one of the many others.

Look at the pinout of 6L6 and EL34 and make a comparison. You will need to wire the 8 pin socket appropriately for the EL34. ** also note whether you power transformer has enough mA's of current capacity to handle a higher current draw of the EL34. If you PT does not have enough current handling capacity, you may not be able to use EL34's?

NOTE: reference to Doug's Plexi 50 schematic and 6L6 vs EL34 wiring

"The only difference between Doug's wiring for the sockets and any standard wiring is that pin 1 (suppressor) is tied to pin 8 (cathode), and therefore pin 1 is not used as a wiring terminal like in Fender amps. This allows the use of EL34's, which have the suppressor on pin 1. 6L6's and 6V6's have beam-forming plates, internally tied to the cathode, which take the place of the suppressor. As a result, they don't even have a pin 1 (or on the ones that do, it is not connected to anything inside the tube).  Fender just used pin 1 on the socket as a convenient wiring terminal."
 
** post on the current hoffmanamp forum with further questions about this

6EU7 has a similar current draw and gain factor as a 12AX7. Look up the pinouts of each and re-wire the socket for a 12AX7.

[tubenit]

25)     How can I find out what each of the pins are on a particular tube like a
         12AX7?

There are many website that list tube data including pinout information. The NJ7P Tube Database is one of those.

12AX7                        pins 1 & 6 are plate
             pins 2 & 7 are grid
             pins 3 & 8 are cathode
             pins 4/5 & 9 are heaters

This applies to any of the 12A_7 type tubes.

[tubenit]

Disclaimer:

Please do not work on tube amps if you do not know how to work on tube amps safely.

The following answers to FAQ's are intended for educational purpose and it is strongly suggested to check other sources for additional information and accuracy. The answers are not intended to promote a particular approach to amp building or repair. It is your responsibility to know how to work on your amp safely and correctly.

It is highly recommended to read Doug's Library of Information first as many answers are listed there as well as other information you will need.

Note: many answers that were posted by tubenit are answers that were cut and pasted from forum posts and/or were simply a reflection of what others have shared. More than not, tubenit simply organized forum information.

[tubenit]

20 part B)  How does someone lacquer tweed for an amp cabinet?

Here is a post that addresses that issue:

http://home.flash.net/~guitars/archive.html#tweed

"Here's the "formula" as discussed on the old TDP, and field tested by Bill Hullett: Orange (amber) shellac cut 50/50 (or so) with denatured alcohol solvent, applied with a bristle brush. Multiple coats to achieve the color, and even out splotchiness. Satin lacquer topcoat if desired for protection from alcohol and moisture, and/or to reduce sheen. Brushing splotches could be reduced by spraying, but look fine and authentic. Practice on scrap first... "

[Platefire]

#26-How do I get started to build my own tube amp?
 
If you think you would like to build your own tube amp and have little or no experiance in electronics, soldering or building an amp cabinet, there are a few basic things to consider as follows:

GETTING STARTED ARTICLE This link goes to the "The Vintage Silvertone Zone", at that point you will have to scroll down the topics on the left hand column of that page and click on "Articles", that will take you to another page and to read the article click on "Building your own Tube Amp, Part 1"
http://www.doctoraudio.us/Silvertone/silvertone.html

X-CHAMP INSTRUCTIONS On the above linked article on the Silvertone Zone, the article refererences my first amp project complete with instructions and layout but the link is no longer attached to the artical so I have attached it below for your information. These instructions was created by CASEY4S for me and anyone that may find this info helpful.
http://briefcase.yahoo.com/bc/casey4s/lst?.dir=/DIY+XChamp+++5F1F2+Champ+Variation&.src=bc&.view=

CONVERTING OLD TUBE AMPS Many get started working on tube amps by buying an old Hi-fi or PA amps and converting them to guitar amps. The following link is a pretty good article written on this subject.
http://www.geofex.com/Article_Folders/old2new.htm

AMP/SPEAKER CABINETS This is a site and forum dedicated to building guitar amp speaker cabinets that I have found very helpful on every aspect of guitar cabinets.
http://www.webervst.com/enclosures/

[dragonworks]

#27 What passive components can be tested in circuit without one side being lifted while amp is shut down.  What components need one side lifted to be tested with amp shut down?

[sluckey]

#27 What passive components can be tested in circuit without one side being lifted while amp is shut down.  What components need one side lifted to be tested with amp shut down?

All of them. None of them. Totally depends on the circuit. If you don't know how to tell from looking at the circuit, then best to lift a lead.