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Post subject: Hotrod deluxe switch to EL34's
Posted: Tue Dec 23, 2008 3:31 pm
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hey all,

just bought a hotrod deluxe a few months ago, is it possible to swap the tubes for EL34's rather than 6L6's., if so how?


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Posted: Wed Dec 24, 2008 4:14 pm
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I think Groove Tubes sell an adapter for that. You might wanna check them out.
CC


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Posted: Wed Dec 31, 2008 4:12 pm
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My amp tech ,who i an authorized fender repairman,told me that the el's are the hottest running tubes there is,even though they are smaller then 6l6's,,,I wouldnt know if it makes a difference,but you would think it wouldnt be all that great for the amp.I have the el's in 2 crate vintage club amps that i am fond of


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Sat May 30, 2015 11:27 pm
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Yes, you can switch to EL34s. You will need to have a voltmeter with mV level accuracy, but the solution is simple. 6L6 and El34s are pin for pin compatible. If you think otherwise, take a look at how easy this is to do with a Mesa.

After you swap the 6L6s with EL34s and rebias (target the same current as you would with 6L6s, 60 mA total which is 30 mA per tube), the job is "done". As long as you play below peak levels of the amp you will be fine. If you like to push the amp, you should be concerned about exceeding screen current. To avoid this, you can switch from the stock 8 ohm speaker to a 16 ohm speaker (to save yourself from yourself), or you can change R61 and R62 (the drop resistors feeding the screens) from 470 ohm resistors to roughly 1k resistors to limit the screen current during high power playing, or take the easy way out and stick a dummy 1/4 connector into the extension jack so the transformer will run off of the 4 ohm tap to power your stock 8 ohm speaker. I would take the easy way out and either put in a 16 ohm version of your favorite speaker or put the dummy connector in place. This will save you many tubes in the long run.

Cheers,

Theaudionut


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Sun May 31, 2015 12:45 am
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theaudionut wrote:
...6L6 and El34s are pin for pin compatible...


No, not quite. Pin 1 of the EL34 is the suppressor grid, and on the 6L6 this suppressor grid is connected internally in the tube to pin 8, and pin 1 is not used on the 6L6.

You could jumper the socket pins 1 and 8 together on the circuit board of the HRDlx, which is done on many amps that use EL34. This is OK on the HRDlx, and you can still switch back to 6L6 because pin 1 is not used on a 6L6, and the pin 1 connection on the circuit board is not used.

You must check each particular amp before connecting anything to the socket, as the unused pin 1 on the socket/circuit board is sometimes used as a connection point for other parts of the circuit.

There is no need to change impedance of the speaker when switching from 6L6 to EL34. There are several amps that use both tube types simultaneously, and either mixed or separate with no impedance switching done. :wink: The only difference is that the negative grid voltage is often changed when switching tube types for proper bias.

Use the same formula for bias setting as with any amp and tube. Max Plate watt rating of the tube, times desired percentage of idle, divided by actual measured Plate voltage, equals current setting. For example, an EL34 rated 25 watts, 70% desired idle watts, and 440 VDC measured at the Plate. (25 x .7)/440 = .040, so about 40 mA per tube. The bias test point on the HRDlx is for two tubes, and in this case mA equals mV at the test point, so using the above example, set the reading at the test point to 80 mV. Use your actual measured Plate voltage and plug into the above formula. I find that EL34 tubes usually sound best at about 70% idle, but you might prefer less, just keep it above 50%.

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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Sun May 31, 2015 1:36 am
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shimmilou wrote:
theaudionut wrote:
...6L6 and El34s are pin for pin compatible...


No, not quite. Pin 1 of the EL34 is the suppressor grid, and on the 6L6 this suppressor grid is connected internally in the tube to pin 8, and pin 1 is not used on the 6L6.

.


+1


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Sun May 31, 2015 1:37 am
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theaudionut wrote:
Yes, you can switch to EL34s.

Theaudionut


Whithout mod ; No !


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Sun May 31, 2015 6:27 am
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stratele52 wrote:
theaudionut wrote:
Yes, you can switch to EL34s.

Theaudionut


Whithout mod ; No !


Have you tried putting in EL34s without mods?


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Sun May 31, 2015 6:36 am
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At any rate, for all the nonbelievers out there I have performed the swap from 6L6 over to EL34s and everything is working fine and the sound is killer. No mods necessary, just a rebias. I hope everybody else out there is now willing to try it.

If I were to do a schematic level mod, it would be changing the screen resistors from 470 to something a little higher to prevent screen damage at high volumes. I'm running a 16 ohm load off of the 8 ohms tap so there is some inherent protection this way to lessen any concerns.

Enjoy.


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Sun May 31, 2015 10:10 am
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theaudionut,

You referenced a MB amp, in which the mod is already done (jumpered pins 1 and 8 ), but you haven't mentioned trying in a HRDlx. Your assumptions are a little presumptuous. :wink:

While it might work in HRDlx without the jumper, the best case scenario is that the EL34 will not be operating as designed to it's full potential.

The part about the impedance mismatch for "protection" is just plain unnecessary (the MB amps that you referenced do not do this), but won't hurt anything. Upping the SGR value is a good thought, although it can compress the sound a bit, but otherwise OK.

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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Sun May 31, 2015 10:33 am
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shimmilou wrote:
theaudionut,

You referenced a MB amp, in which the mod is already done (jumpered pins 1 and 8 ), but you haven't mentioned trying in a HRDlx. Your assumptions are a little presumptuous. :wink:

While it might work in HRDlx without the jumper, the best case scenario is that the EL34 will not be operating as designed to it's full potential.

The part about the impedance mismatch for "protection" is just plain unnecessary (the MB amps that you referenced do not do this), but won't hurt anything. Upping the SGR value is a good thought, although it can compress the sound a bit, but otherwise OK.


Sorry about the assumptions. The swap was made on Hot Rod Deluxe. The reason I reference Mesa Boogie is because there is switch for swapping from 6L6 to EL34. This switch is a DPDT, it does two things: Changes the fixed bias (from about -39v to -51v, I think those are the numbers), and adds a second 470 resistor in series with the other 470 (for a 940 screen resistance). Everything else in the design is almost the same as a Fender power amp section.

By rebiasing the Hot Rod it accomplishes the first requirement. By changing R61 and R62 in the Hot Rod from 470 to 940 or 1k will accomplish the second requirement. If you are careful not to push power levels you can get away with the stock 470 ohm screen resistors. Hence, the hot rod can easily accept EL34s.

If you want more toward class A then sure, bias to 40mA each. Just be careful of power dissipation. Get your voltmeter out and measure across the plate and cathode (or ground, since only mV are lost in the 1 ohm measuring resistor), and multiply this by the current to get a feel for how close the tubes are operating to their spec'd limits.

Pardon any confusion. Have fun installing EL34s in your Hot Rods people! Bias responsibly.


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Tue Jun 02, 2015 7:21 pm
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But I would agree with 'shimmilou' and 'stratele52' regarding the supressor grid on pin 1. By dropping in EL34s the supressor grids (g3, pin 8) are left floating, whereas 6L6s don't have a supressor grid so Fender has this pin unused (electrically, it is used for mechanical support but goes nowhere galvanically). It hasn't posed a problem yet, but then again I've been running at bedroom levels.

More on that...

At low volume the effect of the floating supressor grids is hard to spot. As the volume increases the effect of the floating pin starts to show in the form of early distortion and possibly lower power levels (I can measure this, if requested). To get around this I soldered a very small piece of wire connecting pins 1 and 8 (they are right next to each other, very easy indeed) of the EL34s. Power levels are back up and still no mods were required to the amp. The wire was easy to solder because the pins are adjacent. Be sure to put them as flush against the base of the tube's connector as possible to not impede the socket.

Again, you can still use the EL34s at bedroom levels without concern for g3 (supressor grid) but at live levels you will want that supressor grid tied to the cathode (or ground).


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Tue Jun 02, 2015 7:33 pm
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One other caveat. Pin 1 (6L6GC) in many Fender amps is tied to the control grid, via the grid stopper resistor. You need to remove this resistor off pin 1 and put it onto a terminal strip (or some such attachment), before using the EL34.


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Tue Jun 02, 2015 8:03 pm
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BMW2002Ti wrote:
One other caveat. Pin 1 (6L6GC) in many Fender amps is tied to the control grid, via the grid stopper resistor. You need to remove this resistor off pin 1 and put it onto a terminal strip (or some such attachment), before using the EL34.


Interesting. From what I understand, the function of the supressor grid is to evacuate charge from ions that bounce off of the anode (reflected, not swept out of the space charge region). The more negative the voltage on the supressor grid the more effective it is at removing charge from secondary emissions. Usually the control grid g1 is between -30v and -50v.

Are you sure this resistor needs to be removed? Are there any models that you areaware of that have this connection between g3 and g1? It would be appreciated by any readers to know this specific case. Regarding this thread for both the Hot Rod Deluxe and Hot Rod Deluxe III (and Devilles), pin 1 is isolated so no issues will arise from connecting pin 1 to either ground or cathode, onboard the tube or in the amp. Aside from the theory I agree that there could be other reasons that are amp-specific that must be considered. When in doubt It is always better to err on the side of caution.

Cheers


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Post subject: Re: Hotrod deluxe switch to EL34's
Posted: Tue Jun 02, 2015 8:27 pm
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What BMW2002Ti is referring to, what I mentioned earlier, is that some amps use the pin 1 socket connector as a tie point for other parts of the circuit. So, If you use EL34s, and some other part of the circuit is connected to the socket pin 1 connections, you must remove whatever is connected to the socket pin 1 before plugging in EL34 tubes.

As noted, the HRDlx circuit board has nothing connected to pin 1, so you can simply connect pin 1 of the socket to pin 8 of the socket. I would do it on the circuit side of the board, so no need to remove the board to do it.

The 6L6 does have a type of suppressor grid, namely "beam forming plates" that are connected internally to pin 8, the Cathode. Many schematics simply show these beam forming Plates as a grid, so some 6L6 could actually have a suppressor grid instead of beam forming Plates, I'm not sure.

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