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DT50 Preamp Tubes (one ot two stage preamp)?
by broadfca on 2011-10-29 02:54:14

My DT50 Amp has two (2) preamp tubes. Is this because it's a two (2) stage preamp, or is it one tube for each channel?

If it is a two (2) stage preamp, and I was standing behind the amp looking into it, would stage one (1) be the tube on my left or the tube on my right?

Thanks to all

Re: DT50 Preamp Tubes (one ot two stage preamp)?
by Nick_Mattocks on 2011-10-29 04:15:57

The two 12AX7/ECC83  tubes are not pre-amp tubes in the traditional sense.  They are both part of the power amp section. 

One boosts the overall level of the pre-amp output to drive the EL34's and the other is used as an inverter.


Re: DT50 Preamp Tubes (one ot two stage preamp)?
by broadfca on 2011-10-30 03:57:01

Thank you Nick, your timely response is very much appreciated. Here is the reason for my question:

I play mostly BB King to SRV style music. So far I am very pleased with this DT50. However, the amp is capable of putting out way more distortion than I will ever want. So, I am interested in expirimenting with different tubes for the one that boost the overall level of the preamp output. (Such as the 12AT7EH and the 12AY7EH)

Using my example of standing behind the amp looking in to it, with the amp right side up, would the boost tube be the tube on my right or the tube on the left.

Thanks again for your help.


Re: DT50 Preamp Tubes (one ot two stage preamp)?
by spaceatl on 2011-10-30 07:46:16

The preamp type tubes in the DT50 are NOT preamp tubes...They are part of the power amp...The preamp is a digital modeler and all your preamp distortion is coming from there...

12AT7 and 12AY7 are less gain than 12AX7, but changing them is NOT going to reduce the distortion potential of the preamp....It would likely subtley change the power amp repsonse...good or bad? I don't know...experiementing is the only way to know...

I will have to go take a look at my DT50 112 as I am not sure if I am remembering this correctly...I have a GTSAG-AX7 in there...balanced inverter. I use an Mesa SPAX7 for the buffer tube.

I think the one nearest the speaker jacks is the inverter....I think the head is backwards from the combos but I am not sure...That information is in the forum some place, I just could not located it...

Re: DT50 Preamp Tubes (one ot two stage preamp)?
by broadfca on 2011-11-01 06:57:10

Well...... this water is definitely deeper than my swimming skills are equiped for. Maybe I'll just leave it alone and enjoy waves.

Switching gears a bit.... I just got a Pod HD300 to play with the Link 6 amp modeling features of this amp. This is probabaly a better path for me to find the tones I want.

I was following the Advanced Guide to do the SETUP of the HD300. Everything was going just as the guide showed until I got to the AC rate options. I wanted to be sure it was set at 60Hz. The problem is, the AC rate wasn't there??? Everything else matched the guide exactly.... but NO AC rate showed at all. Any ideas as to why that would be?

Also, am I correct in assuming that when using the Link 6 connection, that the Pod300 should be set for ComboAmp and NOT ComboFrt ?

Lastly, does the Link6 cable need to be unpluged from the Pod300 before   updating via USB ?

These are questions I haven't been able to find the answers to yet.

Thanks for all your help


Re: DT50 Preamp Tubes (one ot two stage preamp)?
by spaceatl on 2011-11-01 07:25:22

Sounds like you need a firmware update for your 300...You can do that with a PC, USB cable and Line 6 Monkey...

Likely, if you have not done it, your DT50 might need to be updated also...For that one you need a midi interface on your PC...You can pick up an M-Audio UNO USB-midi interface for around $30 or so...

Yes, disconnect the link while updating...

In terms of the mode, the most important is the Mode switch being in LIVE mode...I keep my soft output mode set to Combo PWR AMP on my HD400...

Re: DT50 Preamp Tubes (one ot two stage preamp)?
by broadfca on 2011-11-05 07:18:09

Is there any reason to update the DT50 if you're not using a Pod HD "500" ?

I have the HD 300. Will a DT50 update be helpful in this case?



Re: DT50 Preamp Tubes (one ot two stage preamp)?
by Nick_Mattocks on 2011-11-05 07:24:41

Yes - I would say you should update as there may have been some minor changes made to the four on-board HD PRE amp models since your amp's firmware was released.

It's not mission critical that you do this if you aren't using a POD HD of course and if you are happy with your sound as-is you can leave updating alone for now, but nevertheless, me being me, I would update personally


Re: DT50 Preamp Tubes (one ot two stage preamp)?
by broadfca on 2011-11-07 04:10:41

Thanks Nick, went ahead and did the amp update.

I just recently got this amp used from ebay. I noticed yesterday, that when the master volume is turned to full volume, there is a low frequency hum.

With both "channel" volumes turned all the way down, and the "master" turned all the way up.... when I start turning the channel volume up, as I progress to half volume the hum starts becoming clearly audible and gets a little louder as I progress to full channel volume. This happens on both channels, which ever channel the amp is set to ( the channel which is in use).

Anyone have any ideas as to what might cause this?



Re: DT50 Preamp Tubes (one ot two stage preamp)?
by Nick_Mattocks on 2011-11-07 09:14:25

The bias settings might be slightly out of whack or perhaps the tubes need looking at.  However all valve amps emit a low level of background hum and it's normal, but this isn't usually very loud and the level of background hum doesn't normally increase when you turn the amp's volume controls up.  The problem with buying used is that you don't really have full knowledge of an item's history.  

Does this hum present itself with nothing connected to the amp?   I'm just ruling out dodgy cables and ground loop issues.

My gut feeling is telling me you should check the two ECC83/12AX7 valves and perhaps get an amp tech to check the bias of the amp for you.


Re: DT50 Preamp Tubes (one ot two stage preamp)?
by broadfca on 2011-11-08 04:57:54

Thanks Nick. yeah, the amp does this with nothing hooked up to it. That was the first thing I checked. It acts the same with or without my hd300 connected vis L6.

I replaced the 12AX7's with new tubes. That cut down the hiss and noiseiness of the amp considerably, but the hum is still there. From what I'm hearing, a little hum is ok except that it shouldn't get louder as you turn up the volume knobs, as mine does.It's not what I would call a loud hum, and it doesn't project through the guitar sound when I play, but when you stop playing, you can hear the amp sitting there with a low hum coming from it.

Guess I need to find an authorized repair center. However, at lower volumes, below half way, it sounds really good and the hum is not really noticible.

Thanks again Nick

The information above may not be current, and you should direct questions to the current forum or review the manual.