KT88 vs EL34 vs 6L6 Substitution In Vacuum Tube Amplifier.wmv

[essoterric]

It might need new tubes but It is wise to stay with the same output tubes. Your amp is designed for the EL34. ECC83/12AX7's can last a very long time as they run much cooler. Adjusting bias - that's a whole other discussion. Using a good quality contact spray cleaner does miracles and may be all you need. Dirt, grit (and beer) take a serious toll on musical amps. Spray/clean every pot/switch/and plug if you are comfortable with working on your own amp. High voltage in tube amps can be LETHAL.

[SouthernPrinceKenny]

I have a Marshall JCM 900 4100 Hi Gain Dual Reverb that's cutting out and losing volume was squeeling the other day. I think it needs retubing. The preamp tubes are JJ ECC 83.s and the pwr tubes are Mesa EL34 str 447 model...... Can I go to JJ 6l6GC in place of EL34's? How much does it cost to replace the 3 preamp tubes and 4 pwr tubes and biasing nowdays? I think I want to go with all JJ tubes. Thanks for the info.

[essoterric]

6L6's are really good to 40 watts and 50 watts max with any reasonable amount of distortion in the creation of music. If you drive it into a square wave output you may get 60 watts. Similarly, the 6550/KT88 is outstanding at 75 watts but will do 100 watts at higher distortion. Square wave output on a KT88 might reach 120 watts. Just because someone puts 160 MPH on the speedometer of a car does not mean it will go that fast – unless it is off a cliff and it won’t be long before the ride is over.

[JustSomeRandomNewb]

Howcome you dant even gegt 50W with the pair of 6l6 in push pull, while 6l6 in push pull is supposed to do 60?

[MICKSHRED]

Hi, would it be possible to replace my 6L6s with KT88s in my peavy 6505+? What would happen? Thanks

[essoterric]

@MICKSHRED No, that would be a bad idea. As documented in one of my later videos, filament current of the 6L6 is 0.9A and the 6550/KT88 is 1.6A so you will overload your filament winding in the power transformer. Also, the 6L6 runs at about -40 volts whereas the KT88 runs at about -46 volts so the KT88's would be drawing much more cathode current (once again) overloading your power transformer. The other way around (might) be more acceptable.

[tomfoyfx]

I was wondering how well tubes besides EL34's would work in my fixed bias Traynor YSR-1, and this gives me a pretty good answer; thanks.

[RODALCO2007]

Excellent video, very informative.

Thanks for showing.

[mummyjohn]

Seriously? .wmv is on the end of the video title? Did you even LOOK?

[essoterric]

Bias adjustments are meant to set a value of cathode current which is definitely related to plate voltage. The cathode current is the sum of all the currents in the tube (i.e. plate and screen) but the screen current is small so essentially, the cathode and plate current are the same. The Current is what you actually want to set and ultimately you want the tubes to be "matched" by having the same current (measured in mA) flowing in each cathode of the output tubes which makes them "balanced".

[essoterric]

I think over all, that is good advice from Dr Z for most people but I have never been one to just take things at other people's word. I need to understand and find out for myself. I am now 62 and have been this way since I was a small child. I like to know and understand from my own measurements. This attitude carries with it some risk and cost but also the possibility of much real gain. Research your subject well but don't be so timid as to never be able to Plug-It-In and Turn-It-On to learn.

[ossopalatino]

I think that bias adjutaments are related with plate voltage.

[jamesraymondsmith]

hi, love yourr posts ,in the past i have asked dr z "about subbing kt66's in my kt45 (el34s or kt77s) and vice versa with the rte 66, i bias my own amps, i have aplate voltage/bias meter, i would like to try kt88s in in (it has an ef86 , 12ax7 and can use a 5v4, 5ar4, ot solid state rectifier,, i think dr z would rather you buy an amp you want with the tubes you want , rather than mess with his design,, i respect that also.. any ideas ? j

[essoterric]

And lastly, remember that these "experiments" in swapping tubes around are not meant to be permament changes. KT88's are more powerful tubes than the 6L6 or 6CA7 and will require more energy to be delivered by the power supply. Ultimately, my goal was to answer a client's question and my last and final recommendation is to Not swap around your tubes unless you are knowledgable and experienced with working with vacuum tube amps and are willing to accept the risks to yourself and your equipment.

[essoterric]

You should be able to plug your KT88's into your 6CA7 amp (if and only if) you have enough range in your bias adjustment to lower the cathode currents in the KT88's to 70 mA per tube. The socket as wired for the 6CA7 should be OK for the KT88 and as I mentioned below, the worst case would be bias voltage on the metal ring of the KT88's. Check your socket. If pin one is grounded there will be no bias voltage on the metal ring. Work safely with High Voltage.

[essoterric]

I have all of amps wired so that I can do these experiments. If you choose to experiment with your amps you Must know how the sockets are wired on your output tubes. Also, and most important - always practice safety when dealing with high voltage. Unplug the amp and discharge all power supply capacitors. At the lease you can get a really nasty shock which will be most unplesant and you will never forget and at worst it can be LETHAL to you or someone else that touches it. Be Careful...

[essoterric]

So, to make the socket ready to accept either the 6L6/KT88/6CA7 - you Must remove anything soldered to pin 1 of the socket and either ground pin 1 or connect it to pin 8 (the cathode) - probably best to ground it - Connecting pin 1 and pin 8 together carries a certain risk as pin 1 on metal based tubes have the metal ring connected to pin 1 so you could end up with the bias voltage on the metal ring of the tube and it will bite you if you touch it with the amp powered up. see next post

[essoterric]

The 6CA7 filament draws 1.5 A and the KT88 draws 1.6 A. This probably OK as it only 200 mA difference and (probably) will not harm the power transformer. The 6CA7 and KT88 can both take 800 volts on the plate whereas the 6L6 can not. Also, the 6L6 has a filament current of only 0.9 A so putting 6CA7 and KT88 in a 6L6 amp will probably grossly exceed the filament current rating of the power transformer. see next post

[essoterric]

Pin 1 and pin 6 on the 6L6 is not used and often times used as solder connections, on the tube socket, for component connections under the chassis. The 6CA7 pin 1 is the screen grid and is most always grounded or connected to the cathode. The screen grid is internally tied to the cathode in the 6L6. So, you MUST know how the socket is wired up before swapping tubes around. A socket can be wired to take either of these tubes. Check out the pin configurations before swapping tubes. see next post

[TheMoodooM]

So what youre saying is that we can put kt88's in place of el34's, we just have to make bias adjustments to the tubes specs.

[killthebetrayer]

Your video is really good! I learnt many things about tube replacement, and the idea of the BIAS pot on top of the chassis is interesting. I'll try all this in my amps, Thank you ;)

[donnyab]

30 dollars in 77, thats expensive!

[essoterric]

Yes, you should adjust the cathode currents when you change tubes. You do this by measuring the voltage across a cathode resistor (it it has one for this purpose). An example is the Fender DeVille which has a 1 ohm resistor from the 6L6 cathodes to ground and a designated test point which you adjust to 60 mV. This gives 60 mA current for the two tubes. If only a bias voltage point is provided or you measure directly at the grid, it shouldn't have change when tubes were replaced. Hope this helps.

[IdimmuXul]

I'm not as knowledgeable on the subject, but shouldnt you always bias your new tubes no matter what you had before?? or am i misunderstanding what you mean by Direct Replacement?

[preludefan]

very interesting video - and educational, thank you very much

[essoterric]

The Fender Bassman 300 I have a video on has around 750 volts on the plates of the 6550's. Certainly no place to plug in a 6L6 or 6CA7. 450 volts on the plates of the 6L6 is not uncommon and is typical in the old McIntosh amps. I agree with you that for a typical 6L6 style amp, 6.5Kohm output transformer performs well - much better than a 5Kohm. Thanks again.

[essoterric]

I completely agree with you that output transformer impedance is critically important. No doubt this plays into this demonstration. The primary goal of this video was to give some empirical evidence of (some of) what is actually going on when output tubes are swapped with another type. The higher bias voltage on the 6550 (may) allow the smaller tubes to be swapped in without immediate destruction but certainly not recommended for long term use. Thanks for your comments.

[gatomas]

I don't have my tube specks book but the diference on these tubes are the plate resistance I know that you could get up to 70 w from a pair of 6550's and only 50 w from a pair of 6L6's of coure adjusting the bias for AB1 mode but if the output transformer is not matched to the plate resistance that I believe is 6.5 Kohms for the 6L6 you are not going to have optimum efficiency and I don't remember which amp but it had 6L6 with 475 volt plate voltage most run the 6L6 at 350 to 400 plate volts.

[essoterric]

I think it's a personal preference and each tube type seems to have its own charming characteristics but overall, large and small, I am partial to the sound of the EL34/6CA7. Thanks

[peter5524]

WHich tubes sound best?